Disclosed herein is an image encoding/decoding method and apparatus based on transform. An image decoding method includes determining a horizontal transform type and a vertical transform type of a current block, deriving a residual block of the current block by performing inverse transform with respect to the current block based on the determined horizontal transform type and vertical transform type, and reconstructing the current block based on the residual block. The determining of the horizontal transform type and the vertical transform type is performed based on at least one of a width or a height of the current block regardless of an intra prediction mode of the current block, when the current block is in an intra sub-block partitions (ISP) mode.