diff --git a/hyvideo/vae/unet_causal_3d_blocks.py b/hyvideo/vae/unet_causal_3d_blocks.py
index 864f2b1..688e119 100644
--- a/hyvideo/vae/unet_causal_3d_blocks.py
+++ b/hyvideo/vae/unet_causal_3d_blocks.py
@@ -34,12 +34,14 @@
 logger = logging.get_logger(__name__)  # pylint: disable=invalid-name
 
 
-def prepare_causal_attention_mask(n_frame: int, n_hw: int, dtype, device, batch_size: int = None):
-    seq_len = n_frame * n_hw
-    mask = torch.full((seq_len, seq_len), float("-inf"), dtype=dtype, device=device)
-    for i in range(seq_len):
-        i_frame = i // n_hw
-        mask[i, : (i_frame + 1) * n_hw] = 0
+def prepare_causal_attention_mask(
+    n_frame: int, n_hw: int, dtype, device, batch_size: int | None = None
+):
+    indices = torch.arange(1, n_frame + 1, dtype=torch.int32, device=device)
+    indices_blocks = indices.repeat_interleave(n_hw)
+    x, y = torch.meshgrid(indices_blocks, indices_blocks, indexing="xy")
+    mask = torch.where(x <= y, 0, -float("inf")).to(dtype=dtype)
+
     if batch_size is not None:
         mask = mask.unsqueeze(0).expand(batch_size, -1, -1)
     return mask
diff --git a/tests/test_mask_generation.py b/tests/test_mask_generation.py
new file mode 100644
index 0000000..15f4f0b
--- /dev/null
+++ b/tests/test_mask_generation.py
@@ -0,0 +1,54 @@
+import torch
+import time
+
+from hyvideo.vae.unet_causal_3d_blocks import prepare_causal_attention_mask
+
+def prepare_causal_attention_mask_original(n_frame: int, n_hw: int, dtype, device, batch_size: int = None):
+    seq_len = n_frame * n_hw
+    mask = torch.full((seq_len, seq_len), float("-inf"), dtype=dtype, device=device)
+    for i in range(seq_len):
+        i_frame = i // n_hw
+        mask[i, : (i_frame + 1) * n_hw] = 0
+    if batch_size is not None:
+        mask = mask.unsqueeze(0).expand(batch_size, -1, -1)
+    return mask
+
+
+def test_mask_generation(n_frame: int, n_hw: int, batch_size: int = 1):
+    dtype = torch.bfloat16
+    device = torch.device("cuda") if torch.cuda.is_available() else torch.device("cpu")
+    original_mask = prepare_causal_attention_mask_original(n_frame, n_hw, dtype, device, batch_size)
+    new_mask = prepare_causal_attention_mask(n_frame, n_hw, dtype, device, batch_size)
+    torch.testing.assert_close(original_mask, new_mask)
+
+def benchmark_mask_generation(n_frame: int, n_hw: int, batch_size: int = 1):
+    def run_benchmark(fun, n_warmup: int = 2, n_iter: int = 5):
+        for _ in range(n_warmup):
+            fun()
+        
+        if device.type == "cuda":
+            torch.cuda.synchronize()
+        t0 = time.time()
+        for _ in range(n_iter):
+            fun()
+        if device.type == "cuda":
+            torch.cuda.synchronize()
+        return (time.time() - t0) / n_iter
+    
+    dtype = torch.bfloat16
+    device = torch.device("cuda") if torch.cuda.is_available() else torch.device("cpu")
+
+    t_orig = run_benchmark(
+        fun=lambda: prepare_causal_attention_mask_original(n_frame, n_hw, dtype, device, batch_size)
+    )
+    t_new = run_benchmark(
+        fun=lambda: prepare_causal_attention_mask(n_frame, n_hw, dtype, device, batch_size)
+    )
+
+    print(f"Original mask generation time: {t_orig:.4f}s")
+    print(f"New mask generation time: {t_new:.4f}s")
+    print(f"Speedup: {t_orig / t_new:.2f}x")
+
+if __name__ == "__main__":
+    test_mask_generation(64, 128, 1)
+    benchmark_mask_generation(32, 1024, 1) # 45x speedup on GPU