Abstract: Objective: To study the accuracy between CT and MRI in delineating gross tumor volume (GTV) of nasopharyngeal carcinoma (NPC) when making radiotherapy plan. Methods: Thirty- nine cases with nasopharyngeal carcinoma confirmed by pathology, selected during the period from April 2003 to April 2004, were reviewed retrospectively. CT scanning was performed with GELight speed 16,routine axial scan was parallel to the OM line from soft palate to the suprasellar cistern. MR scanning was performed by GE Signa super- conducting magnetic resonance imaging system (1.5 Tesla). All data were analyzed using the paired- samples t test. Results: The mean primary tumor volume (cm3) of CT Group and MR Group were 32.49 ±19.91 and 29.06±8.75 respectively, and the difference between the two groups was significant (t=5.268, P=0.000). There were significant differences in the two groups at early stage and advanced stage by Fuzhou Staging System (t=5.677, P=0.000; t=3.310, P=0.005, respectively). There was significant difference in Stage T1, T2, T3 (P=0.005, P=0.001, P=0.004, respectively ),but not in stage T4 (P=0.146). Conclusion: M R is more accurate than CT in delineating GTV of NPC,so it is more valuable in making radiotherapy plan.