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; 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, when a prediction mode is an inter prediction mode, not an intra prediction mode, a size of the at least one prediction unit in the coding unit is determined independently from a size of the at least one transformation unit in the coding unit, the image is split into a plurality of maximum coding units, a maximum coding unit, from among the plurality of maximum coding units, is hierarchically split into one or more coding units of depth including at least one of a current depth and a lower depth, 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 the lower depth, independently from neighboring coding units, and when the split information indicates a non-split of the lower depth, the at least one prediction unit is obtained from the coding unit of the lower depth and the at least one transformation unit is obtained from the coding unit of the lower depth.