Abstract: Objective To observe the reversal effects of cyclooxygenase-2(COX-2) selective inhibitor NS-398 on the multi-drug resistance of colorectal cancer cell line HCT-8/Fu and its regulatory effects on the expression of P-glycoprotein(P-gp), multi-drug resistance related protein(MRP), in order to explore its mechanism. Methods (1)The inhibition rate and the reversal of multi-drug resistance(MDR) of HCT-8/Fu cells by NS-398 were detected by CCK-8 kit; (2)NS-398, 5-Fu or NS-398 combined with 5-Fu were added on HCT-8/Fu cells for 24h respectively. Caspase-3 activity was analysed by automatic fluorescence enzymelinked immunoassay detector; the cells were stained by Hoechst33342 and observed under fluorescent inverted microscope; (3)HCT-8/Fu cells were treated by NS-398(0, 10 and 20μmol/L) for 24h, ELISA analysis were performed to detect PGE2 concentration in the culture supernatant; and the expression of P-gp, MRP before and after 20μmol/L NS-398 treatment were detected by immunocytochemical staining. Results (1)The inhibition rates were 6% and 8% respectively after HCT-8/Fu cells were treated by 10 and 20 μmol/L NS-398 for 24 h; when combined with various concentrations of 5-Fu, the reversal effects were 3.42 and 7.50 times respectively, in a dose-dependent manner. (2)Caspase-3 activity of the combined group(20μmol/L NS-398+320μg/ml 5-Fu) was increased significantly by 386.11%, compared with 179.94% in 320 μg/ml 5-Fu group and 125.23% in 20 μmol/L NS-398 group; the morphological changes by Hoechst33342 staining were consistent with this result;(3) The expression of P-gp and MRP were decreased significantly after treated by 20 μmol/L NS-398 for 24h detected by immunocytochemical staining. Meanwhile, when HCT-8/Fu cells were treated with 10 and 20 μmol/L NS-398 for 24h, the levels of PGE2 were 189.50 and 151.25 ng/L respectively, significantly lower than 340.13 ng/L PGE2 treated with 0μmol/L NS-398. Conclusion COX-2 selective inhibitor NS-398 could reverse multi-drug resistance of colorectal HCT-8/Fu cells, in a dose-dependent manner.The reversal mechanism may be through inhibiting the expression of P-gp, MRP and the level of PGE2.