A method and apparatus for encoding/decoding an image that divide an image sequence into sub-groups and determine encoding modes applied to bi-directional pictures included in each sub-group using correlations between the bi-directional pictures and reference pictures are provided. The image encoding method includes dividing a group of pictures (GOP) to be encoded according to consecutive B pictures into sub-groups; calculating the correlations between each B picture included in the sub-group and reference picture according to each encoding mode; and applying one of the encoding modes wherein the reference picture having the highest correlation with the B picture are used in each sub-group to encode the image, thereby improving encoding efficiency.