A method (500, 600) for processing chroma blocks by a decoder, comprising: partitioning (510, 610) a coding block into a plurality chroma blocks; merging (520, 620) at least two chroma blocks of the plurality chroma blocks into a merged block, wherein a width or height of one of at least two chroma blocks is less than a threshold, and a width or height of the merged block is no less than the threshold; deriving (530, 630) prediction information for the merged block; generating (530, 630) prediction samples according to the derived prediction information; and obtaining (540, 650, 660) reconstructed chroma samples according to the generated prediction samples; wherein if the chroma blocks inside the merged block are all intra-predicted blocks, the deriving prediction information for the merged block comprises: identifying an intra prediction mode used by one of the at least two chroma blocks, wherein the intra prediction mode is DC mode, planar mode, or an angular prediction mode, and using the identified intra prediction mode as the intra prediction mode for the merged block; or wherein if the chroma blocks inside the merged block are all inter-predicted blocks, the prediction information comprises a motion vector and a reference frame for the merged block, wherein the motion vector and the reference frame for the merged block are copied from a motion vector and a reference frame of one chroma block inside the merged block, or the motion vector and the reference frame for the merged block are derived from motion vectors and reference frames of chroma blocks inside the merged block; or wherein if the chroma blocks inside the merged block include an intra-predicted block and an inter-predicted block, the merged block is inter predicted or intra predicted, a prediction mode for a largest block which contains most number of pixels inside the merged block is identified and used as the prediction mode of the merged block, or a prediction mode for a first block in coding scan order inside the merged block is identified and used as the prediction mode of the merged block. partitioning (510, 610) a coding block into a plurality chroma blocks; merging (520, 620) at least two chroma blocks of the plurality chroma blocks into a merged block, wherein a width or height of one of at least two chroma blocks is less than a threshold, and a width or height of the merged block is no less than the threshold; deriving (530, 630) prediction information for the merged block; generating (530, 630) prediction samples according to the derived prediction information; and obtaining (540, 650, 660) reconstructed chroma samples according to the generated prediction samples; wherein if the chroma blocks inside the merged block are all intra-predicted blocks, the deriving prediction information for the merged block comprises: identifying an intra prediction mode used by one of the at least two chroma blocks, wherein the intra prediction mode is DC mode, planar mode, or an angular prediction mode, and using the identified intra prediction mode as the intra prediction mode for the merged block; or wherein if the chroma blocks inside the merged block are all inter-predicted blocks, the prediction information comprises a motion vector and a reference frame for the merged block, wherein the motion vector and the reference frame for the merged block are copied from a motion vector and a reference frame of one chroma block inside the merged block, or the motion vector and the reference frame for the merged block are derived from motion vectors and reference frames of chroma blocks inside the merged block; or wherein if the chroma blocks inside the merged block include an intra-predicted block and an inter-predicted block, the merged block is inter predicted or intra predicted, a prediction mode for a largest block which contains most number of pixels inside the merged block is identified and used as the prediction mode of the merged block, or a prediction mode for a first block in coding scan order inside the merged block is identified and used as the prediction mode of the merged block. identifying an intra prediction mode used by one of the at least two chroma blocks, wherein the intra prediction mode is DC mode, planar mode, or an angular prediction mode, and using the identified intra prediction mode as the intra prediction mode for the merged block; or wherein if the chroma blocks inside the merged block are all inter-predicted blocks, the prediction information comprises a motion vector and a reference frame for the merged block, wherein the motion vector and the reference frame for the merged block are copied from a motion vector and a reference frame of one chroma block inside the merged block, or the motion vector and the reference frame for the merged block are derived from motion vectors and reference frames of chroma blocks inside the merged block; or wherein if the chroma blocks inside the merged block include an intra-predicted block and an inter-predicted block, the merged block is inter predicted or intra predicted, a prediction mode for a largest block which contains most number of pixels inside the merged block is identified and used as the prediction mode of the merged block, or a prediction mode for a first block in coding scan order inside the merged block is identified and used as the prediction mode of the merged block.