The gamma-GT activity in the serum and pharynx tissue of both NPC patients (106) and pharyngitis patients (65) were detected with method of hydrolysis gamma-glutamylnitrophenylamine. The 34 NPC patient's gamma-GT were assayed both before and after radiotherapy. The results indicated that the gamma-GT activities between the NPC patients and pharyngitis patients, and the activities before and after radiotherapy have significant differences. The investigation suggests that the determination of serum and pharynx tissue gamma-GT levels of NPC patients would help to diagnose and evaluate the curative effect for NPC.