A method of encoding and decoding image data, and an apparatus to perform the method, are provided. The image data encoding method includes: converting and quantizing pixel values of a block of image data to form a conversion block of image data; determining a classification mode used to classify the conversion block into a first region having one or more coefficients other than 0, and a second region having all coefficients of 0, based on a diagonal of the conversion block; and generating bit streams for the coefficients of the first region according to the determined classification mode and a first bit depth indicating a bit number required to binarize coefficients of the conversion block. The image data encoding method can increase the compression rate and perform encoding in real time while not degrading an image visibly.