diff --git a/cflearn/modules/optimizers.py b/cflearn/modules/optimizers.py
index 0ec8e41d0..399b40cab 100644
--- a/cflearn/modules/optimizers.py
+++ b/cflearn/modules/optimizers.py
@@ -1,3 +1,4 @@
+import math
 import torch
 
 from typing import *
@@ -133,4 +134,117 @@ def step(self, closure: Optional[Callable[[], float]] = None) -> Optional[float]
         return loss
 
 
+@register_optimizer("ranger")
+class Ranger(Optimizer):
+    def __init__(
+        self,
+        params: Iterable[torch.Tensor],
+        lr: float,
+        alpha: float = 0.5,
+        k: int = 6,
+        n_sma_threshold: float = 5.0,  # 4.0
+        betas: Tuple[float, float] = (0.95, 0.999),  # (0.90, 0.999)
+        eps: float = 1e-5,
+        weight_decay: float = 0.0,
+        use_gc: bool = True,
+        gc_conv_only: bool = False,
+    ):
+        defaults = dict(
+            lr=lr,
+            alpha=alpha,
+            k=k,
+            n_sma_threshhold=n_sma_threshold,
+            betas=betas,
+            eps=eps,
+            weight_decay=weight_decay,
+            step_counter=0,
+        )
+        super().__init__(params, defaults)
+        self.n_sma_threshold = n_sma_threshold
+        self.alpha = alpha
+
+        self.radam_buffer = [[None, None, None] for _ in range(10)]
+        self.use_gc = use_gc
+        self.gc_gradient_threshold = 3 if gc_conv_only else 1
+
+    def step(self, closure: Optional[Callable[[], float]] = None) -> Optional[float]:
+        loss = None
+        if closure is not None:
+            loss = closure()
+
+        for group in self.param_groups:
+            for p in group["params"]:
+                if p.grad is None:
+                    continue
+                grad = p.grad.data.float()
+                if grad.is_sparse:
+                    msg = "Ranger optimizer does not support sparse gradients"
+                    raise RuntimeError(msg)
+
+                p_data_fp32 = p.data.float()
+                state = self.state[p]
+                if len(state) != 0:
+                    state["exp_avg"] = state["exp_avg"].type_as(p_data_fp32)
+                    state["exp_avg_sq"] = state["exp_avg_sq"].type_as(p_data_fp32)
+                else:
+                    state["step"] = 0
+                    state["exp_avg"] = torch.zeros_like(p_data_fp32)
+                    state["exp_avg_sq"] = torch.zeros_like(p_data_fp32)
+                    state["slow_buffer"] = torch.empty_like(p.data)
+                    state["slow_buffer"].copy_(p.data)
+
+                exp_avg, exp_avg_sq = state["exp_avg"], state["exp_avg_sq"]
+                beta1, beta2 = group["betas"]
+
+                if grad.dim() > self.gc_gradient_threshold:
+                    grad.add_(-grad.mean(dim=tuple(range(1, grad.dim())), keepdim=True))
+
+                state["step"] += 1
+
+                exp_avg_sq.mul_(beta2).addcmul_(grad, grad, value=1.0 - beta2)
+                exp_avg.mul_(beta1).add_(grad, alpha=1.0 - beta1)
+
+                buffered = self.radam_buffer[int(state['step'] % 10)]
+
+                if state["step"] == buffered[0]:
+                    n_sma, step_size = buffered[1], buffered[2]
+                else:
+                    buffered[0] = state["step"]
+                    beta2_t = beta2 ** state["step"]
+                    n_sma_max = 2.0 / (1.0 - beta2) - 1.0
+                    n_sma = n_sma_max - 2.0 * state["step"] * beta2_t / (1.0 - beta2_t)
+                    buffered[1] = n_sma
+                    if n_sma <= self.n_sma_threshold:
+                        step_size = 1.0 / (1.0 - beta1 ** state["step"])
+                    else:
+                        step_size = math.sqrt(
+                            (1.0 - beta2_t) * (n_sma - 4.0)
+                            / (n_sma_max - 4.0) * (n_sma - 2.0)
+                            / n_sma * n_sma_max
+                            / (n_sma_max - 2.0)
+                        ) / (1.0 - beta1 ** state["step"])
+                    buffered[2] = step_size
+
+                if group["weight_decay"] != 0:
+                    p_data_fp32.add_(
+                        p_data_fp32,
+                        alpha=-group["weight_decay"] * group["lr"],
+                    )
+
+                if n_sma <= self.n_sma_threshold:
+                    p_data_fp32.add_(exp_avg, alpha=-step_size * group["lr"])
+                else:
+                    denom = exp_avg_sq.sqrt().add_(group["eps"])
+                    p_data_fp32.addcdiv_(exp_avg, denom, value=-step_size * group["lr"])
+
+                p.data.copy_(p_data_fp32)
+
+                if state["step"] % group["k"] == 0:
+                    slow_p = state["slow_buffer"]
+                    slow_p.add_(p.data - slow_p, alpha=self.alpha)
+                    p.data.copy_(slow_p)
+
+        return loss
+
+
 __all__ = ["optimizer_dict", "register_optimizer"]