Accurate segmentation of the prostate gland and its zones is a challenging task due to the high variability of prostatic anatomic structures. Gland and zonal segmentation of prostate is a useful tool for computer-aided diagnosis of prostate cancer because characteristics of cancer in prostate zones differ significantly. In this study, we used an active contour model for prostate gland segmentation and atlas-based approach for prostate zonal segmentation in diffusion-weighted MR imaging. We have assessed the performance of segmentation methods using different similarity parameters. The proposed methods are highly robust and show relatively good performance compared to previously reported work.