1. A method of decoding an image, the method comprising: performing entropy-decoding to obtain quantized transformation coefficients of at least one transformation unit in a coding unit of the image; determining a prediction mode of at least one prediction unit in the coding unit from information indicating the prediction mode for the at least one prediction unit; when the prediction mode is determined to be an inter prediction mode, not in an intra prediction mode, determining a size of the at least one transformation unit in the coding unit regardless of a size of the at least one prediction unit in the coding unit; performing inverse-quantization and inverse-transformation on the quantized transformation coefficients of the at least one transformation unit to obtain residuals; and performing inter prediction for at least one prediction unit in the coding unit to generate a predictor and restoring the image by using the residuals and the predictor, wherein: the image is split into a plurality of maximum coding units, according to information about a maximum size of the coding unit, the maximum coding unit is hierarchically into one or more coding units of depth including at least one of a current depth and a lower depth, according to split information, when the split information indicates a split for the current depth, the coding unit of the current depth is split into four rectangular coding units of a lower depth, independently from neighboring coding units, and when the split information indicates a non-split of the current depth, the coding unit of the current depth is split into the at least one prediction unit.