Provided are a method and apparatus for encoding a video and a method and apparatus for decoding a video. The encoding method includes: splitting a picture of the video into one or more maximum coding units that are coding units having a maximum size; encoding the picture based on coding units according to depths which are obtained by hierarchically splitting each of the one or more maximum coding units according to depths in each of the one or more maximum coding units, determining coding units according to coded depths with respect to each of the coding units according to depths, and thus determining coding units having a tree structure; and outputting data that is encoded based on the coding units having the tree structure, information about the coded depths and an encoding mode, and coding unit structure information indicating a size and a variable depth of a coding unit.