A method and apparatus for block partition are disclosed. If a cross-colour component prediction mode is allowed, the luma block and the chroma block are partitioned into one or more luma leaf blocks and chroma leaf blocks. If a cross-colour component prediction mode is allowed, whether to enable an LM (Linear Model) mode for a target chroma leaf block is determined based on a first split type applied to an ancestor chroma node of the target chroma leaf block and a second split type applied to a corresponding ancestor luma node. According to another method, after the luma block and the chroma block are partitioned using different partition tress, determine whether one or more exception conditions to allow an LM for a target chroma leaf block are satisfied when the chroma partition tree uses a different split type, a different partition direction, or both from the luma partition tree.