Provided are an encoding method and apparatus, and a decoding method and apparatus for adaptively selecting a context model used to entropy-encode and entropy-decode a syntax element, based on various shapes of coding units. The image decoding method includes: determining a context model based on block shape information including at least one of a shape, direction, width, ratio of width and height, or size of a coding unit; obtaining, from a bitstream based on the context model, information about a split shape mode for splitting the coding unit; and determining a split shape mode of the coding unit, based on the information about the split shape mode.