Objective To analyze the correlation between primary tumor volume (PTV) and prognosis of nasopharyngeal carcinoma (NPC) treated by intensity-modulated radiotherapy (IMRT). Methods 330 NPC patients treated by IMRT were included. Pretreatment computerized tomography image were input into tree-dimensional treatment-planning system, in which the primary tumor volume were calculated automatically. The receiver operating characteristic curve was used to determine the best cut-off point of PTV. Within the framework of UICC 2002 T stage,The PTV　was divided into four groups:V₁<10 cm³, V₂ 10－25 cm³, V₃>25－50 cm³ and V₄>50 cm³. Kaplan-Meier and Logrank test was used to analyze the survival, Cox proportion risk regression model were used to analysis the correlation between PTV and prognosis. Results The mean PTV for all NPC patients was (34.2±27.1) cm³ with the range of 0.4－153.7 cm³. The 3-year overall survival for V₁, V₂, V₃ and V₄ stage were 88.6%,90.0%,91.2% and 74.2%, respectively (χ²=12.83,P=0.005). There was no significant difference among V₁, V₂ and V₃ in terms of overall survival (χ²=1.96,P=0.376). The 3-year distant metastasis-free survival and diseases-free survival or overall survival were decrease in PTV>50 cm³ and PTV≤50 cm³ (77.4%∶89.9%, χ²=7.24, P=0.007and 64.5%∶85.1%, χ²=13.95,P=0.000 or 74.2%∶90.3%, χ²=11.76,P=0.001). Multivariate analysis revealed that PTV was a adverse prognostic factors for overall survival (χ²=0.00,P=2.580). Conclusion Our data showed that the primary tumor volume had significantly impacted on the prognosis of NPC patients treated by intensity modulated radiotherapy.