Interleaved EPI (iEPI) with a 2D navigator is an effective way to acquire high-resolution and less-distorted DWI. The inter-shot phase variations can be corrected with acquired 2D navigator. However, the performance is limited by the geometric distortion mismatch between the navigator-echo and image-echo. Parallel imaging can be used to reduce the navigator distortion, but can cause noise amplification. Previously, the effects of the navigator were studied with in vivo experiments, but the effects of distortion and noise were mixed together. In this study, the effects of navigator distortion and noise are studied individually with both DWI and DTI simulations.