Objective: To evaluate the precision of image registration between MRI simulation (MRIsim) and CT simulation compared to diagnostic MRI(MRIdiag) and to provide information for further application of MRIsim. Methods: A total of 24 patients who underwent both MRIsim and MRIdiag were enrolled, including 8 patients with gliomas, 8 with nasopharyngeal carcinomas and 8 with prostate cancers. MRIsim and MRIdiag images of each patient were fused with CT. The OARs were delineated on three modalities of images and targets were delineated on fusion image of MRIsim with CT (F_CTMsim) and fusion image of MRIdiag with CT (F_CTMdiag) respectively. The concordance index (CI), Dice′s similarity coefficient (DSC) between the OARs and image similarity index (S) based on images from MRIsim, MRIdiag and CT were evaluated. IMRT plans were designed based targets on F_CTMsim and OARs on CT images, and differences in dosimetry of targets and OARs were evaluated subsequently. Results: Volumes of most OAR from three modalities of images showed no statistically significant difference(P>0.05). All the CI and DSC between the OARs derived from MRIsim and CT were higher than those corresponding values from MRIdiag, and a statistically significant difference was achieved in 50% of these OARs (t=2.58-5.47, P<0.05). The S values of MRIsim and MRIdiag compared with CT were 0.89 and 0.83 respectively (t=5.77, P<0.05). MRIsim improved the S value by 10% (2%-56%) compared with MRIdiag. No further differences in dosimetry were found on all OARs and all targets(P>0.05). Conclusions The precision of image registration can be significantly improved by introducing MRIsim into radiotherapy planning design compared with MRIdiag. However, no significant differences in dosimetry were found on targets produced by rigid registration and manual adjustment method .