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 in 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.