Motion compensation, which usually includes range alignment and phase compensation, plays a key role in ISAR imaging. As the basis of phase compensation, range alignment has been investigated comprehensively and multifarious range alignment methods have been proposed. When the impact of range alignment cannot be judged directly, it is necessary to choose the optimal range alignment method. An evaluation method based on variance of range bins is proposed. It makes use of weighted sum of range bins' variance from range alignment to evaluate impact of range alignment method directly. The results of simulation verify the effectiveness of the method.