Abstract: Objective 　To investigate preliminarily the expression of p-glycoprotein ( P-gp) in patients with nasopharyngeal squamous carcinoma (NPSC) pre- and post-radiotherapy. Methods 　The immunohistochemistry with one monoclonal antibody JSB-1 staining was performed to test the expression of pgp in pre- and post-radiotherapy NPSC specimens from 48 patients . Results 　The positive rate of P-gp expression in the patients with NPSC pre-radiotherapy was 18. 75 %(9/ 48) . Among them, the positive expression rate was 17. 65 %(6/ 34) and 21. 43 %(3/ 14) in low grade differentiation squamous cell carcinoma and mid-high grade differentiation squamous cell carcinoma, respectively. The positive expression rate of post-radiotherapy was 75. 00 %(36/ 48) and among them, the positive expression rate was 79. 41 %(27/34) 、64. 29 %(9/ 14) in low grade differentiation squamous cell carcinoma and mid2high grade differentiation squamous cell carcinoma, respectively. P-gp expression rate post-radiotherapy was significantly higher than that of pre-radiotherapy ( P < 0. 001) . Conclusion 　Our study indicates that pgp expression in NPSC was significantly induced by radiation and increases the overexpression of P-gp. Therefore, irradiation may be one of the reasons of crossed resistance to chemotherapy and radiotherapy.