A novel approach to adapt the macroblock (MB) coding order of chrominance planes is proposed to improve the compression performance of the H.264/AVC intra coder. The proposed algorithm first encodes the luminance plane of an image and then uses the luminance data to determine the MB coding order of the chrominance planes adaptively. The proposed algorithm consists of three steps. First, we estimate the similarities between neighboring chrominance MBs using the luminance gradients. Second, we assign a high priority to a chrominance MB, if it is similar to many neighboring MBs. Finally, we encode the chrominance MBs in the decreasing order of the priorities. Moreover, we develop efficient submodes, representing prediction directions, to exploit the correlations of the MBs along the adaptive scan order. Experimental results show that the proposed algorithm yields significantly better compression performance than H.264/AVC on both YUV 4:4:4 and RGB 4:4:4 images.