A method of decoding an image includes the steps of restoring a residual value by performing inverse quantization and inverse transform on the residual value by entropy decoding a received bit stream, generating a prediction unit by performing intra prediction selectively using one of a plurality of prediction modes on a prediction unit split by conducting at least one of asymmetric partitioning and geometrical partitioning, and restoring an image by adding the residual value to the prediction unit. It may be possible to enhance encoding efficiency of high-resolution images having a resolution of HD or higher by performing intra prediction on the asymmetric partitioning and/or geometrical partitioning.