Provided are methods and apparatus for encoding and decoding motion information. The method of encoding motion information includes: obtaining a motion information candidate by using motion information of prediction units that are temporally or spatially related to a current prediction unit; adding, when the number of motion information included in the motion information candidate is smaller than a predetermined number n, alternative motion information to the motion information candidate so that the number of motion information included in the motion information candidate reaches the predetermined number n; determining motion information with respect to the current prediction unit from among the n motion information candidates; and encoding index information indicating the determined motion information as motion information of the current prediction unit.