Background and Purpose: To establish the minimally required margins in different directions measured from GTV in the definitely treatment of nasopharyngeal carcinoma(NPC) using IMRT based on the 5-year results.Methods and materials: Between November 2003 and May 2007, 414 patients with non-metastatic NPC were treated with IMRT according to our institutional protocol.Treatment outcomes at 5 years were analyzed.Distances from GTV-T to CTV2 (i.e., CTV 59.4Gy) in 6 directions (anterior, posterior, superior,inferior, and bilateral) were measured and analyzed.Results: The 5-year estimated overall survival (OS), disease free survival (DFS), local control (LC) were 80%, 77% and 95%, respectively.For the margins measured from GTV-T to CTV2, margins used with T4 disease were significantly and uniformly smaller than the whole group in all the 6 directions (P=-0.000, 0.000, 0.000, 0.000 and 0.046, respectively).However, no increase of local recurrence was associated to this limited margins used.Conclusions: Our 5-years experience showed a very high LC rate.The strategy we used for CTV delineation was safe and reliable.Determined CTV through GTV expansion to a minimally required margin, using GTV + margin (used in our T4 patients) + the whole nasopharyngeal mucosa, especially for the patients with early T disease, might be feasible.