In EPI-based diffusion imaging, the geometric distortions become more severe with increased image resolution. Various post-processing methods have been proposed to correct for distortions, such as the top-up method and the field-mapping method. Nonetheless, for 3D isotropic high-resolution diffusion imaging, distortion correction becomes more challenging due to decreased SNR. In this study, we applied a modified distortion correction method, which was previously proposed for 2D imaging, to isotropic high-resolution diffusion imaging using 3D simultaneous multi-slab (SMSlab) acquisition. The modified distortion correction method performed well in both phantom and in vivo experiments. It also outperformed the conventional top-up and field-mapping methods.