Abstract: Objective: To investigate the mechanism behind campotothecin (CPT) enhancement of TRAIL-induced apoptosis of PANC- 1 pancreatic carcinoma cell line. Methods:The growth inhibition rates of PANC- 1 cells in the TRAIL, CPT and CPT+TRAIL groups were assayed by MTT. Cellular apoptosis in each group was tested by fluorescent staining with Hoechst 33258 . The level of apoptotic signal transduction proteins, such as Caspase- 8, Caspase- 3, DR4, DR5 and c-Flip, were detected using Western blot. Results: Twenty-four hours after PANC- 1 cells were treated with TRAIL of various concentrations (10, 30, 100 , 300 , and 1000 ng/mL), there were significant differences between the treatment and control groups at 100 ng/ml and higher ( P<0.01). Despite these differences, the inhibition rate was only 10.60 ± 2.36% with a TRAIL concentration of1000ng/mL. There were significant differences between the TRAIL+CPT combination group ( 10, 30, 100 , 300 , 1000ng/mL) and the CPT alone group (P<0.05). Results of Hoechst 33258 fluorescent staining showed that typi-cal characteristics of apoptosis such as nucleus aggregation, marginalization and nuclear fragmentation occurred in the TRAIL+CPT group. However, no obvious signs of apoptosis were seen in the groups treated only with TRAIL. Western blot results showed that pro-caspase- 8 and pro-caspase- 3 could only be seen in the groups treated with TRAIL or CPT alone. However, bands for both the precursors and the cleaved products were seen in the groups treated with both drugs. In addi-tion, the expression of c-Flip protein in PANC- 1 cells was down-regulated after CPT treatment. There were significant differ-ences in the expression of c-Flip protein between the CPT and combination treatment group and the groups treated with TRAIL alone and the control groups. There were no significant differences in the expression of DR4 and DR5 before and af -ter the CPT treatment. Conclusion:CPT combined with TRAIL may have a synergistic effect on induced apoptosis in PANC-1 cells. CPT down-regulates the expression of c-Flip protein, may possibly reverse the inhibition of caspase- 8, and can activate the apoptotic signaling pathway. DR4 and DR5 do not play significant roles.