Objective To analyze the prognosis of masticator space involvement in NPC and evaluate the value of masticator space involvement in T-staging. Methods All the biopsy-proven newly diagnosed non-metastatic NPC 442 patients were collected. Medial pterygoid muscle involvement were classified as stage T3 group. Masticator space (except for medial pterygoid muscle) were classified as stage T4 group. Skull base, intracranial and paranasal sinuses involvement were used as a control group. The following endpoint were assessed:overall survival rate (OS), local recurrence-free survival (LRFS), distant metastasis-free survival (DMFS). Using Kaplan-Meier method, the differences were compared using the Log-Rank test. A two-sided test, P<0.05 had significant difference. Results Overall survival of T3 medial pterygoid muscle group and T4 masticator space except for the medial pterygoid muscle group was statistically significant (P=0.02), local recurrence-free survival and distant metastasis-free survival rates were not statistically significant between them(P=0.55, P=0.09). overall survival, distant metastasis free survival rate of T3 medial pterygoid muscle group and T4 of intracranial sinus group were statistically significant (P=0.02, P=0.01). Overall survival rate, local relapse-free survival and distant metastasis-free survival were not statistically significant between T3 medial pterygoid muscle group and T3 of skull base (P>0.05). Overall survival, distant metastasisfree survival rate of staging T3 and T4 were statistically significant (P=0.001, P=0.004). Overall survival rate, local relapse-free survival and distant metastasis-free survival rate of clinical-stage III and IVa were statistically significant (P=0.002, P=0.025, P=0.003). Conclusion Masticator space involvement in NPC influenced the rate of overall survival, and if the infiltration of masticator space is different, the prognosis is different.