Provided is an image encoding or decoding method and apparatus for post-processing an intra- or inter-predicted block based on pixel gradients. In the image decoding method and apparatus according to an embodiment, prediction mode information of a current block to be decoded and filtering information indicating whether to filter a predicted block are obtained from a bitstream, a first predicted block of the current block is generated based on the prediction mode information, a second predicted block is generated by changing a pixel value of a current pixel in the first predicted block based on gradient values between the current pixel and adjacent pixels of the current pixel, when the filtering information indicates to filter the first predicted block, a residual corresponding to a difference value between the current block and the second predicted block is reconstructed by extracting the residual from the bitstream, and the current block is decoded by adding the residual to the second predicted block.