A graphics processing unit includes a pixel shader, an output merger, a cache, and a memory. The pixel shader is configured to output a pixel data. The output merger is coupled to the pixel shader and configured to receive the pixel data. The output merger outputs the pixel data and a sample mask corresponding to the pixel data. The cache is coupled to the output merger and configured to receive the pixel data and the sample mask. The cache generates a sample data according to the pixel data and the sample mask. The memory is coupled to the cache. The cache writes the sample data into the memory. A data size of the sample data is a multiple of a data size of the pixel data. An operation method thereof is also provided.