The molecular mechanisms and pathogenesis of chronic hepatitis C (CHC) infection are unclear. Innate immune cells such as natural killer (NK) cells and dendritic cells are responsible from molecular mechanism of CHC. NK cell cytotoxicity such as TRAIL expression is important pathway for viral clearance. The aim of this study was to evaluate the relationship between HCV RNA and sTRAIL levels during the first 12 weeks of Peg-IFN alpha and ribavirin treatment. Twelve treatment naive patients with CHC treated with Peg-INF alpha and ribavirin were included in this study. Circulating sTRAIL and HCV RNA levels were measured at baseline, 4th and 12th week of treatment and their correlation was investigated. sTRAIL and HCV RNA levels decreased gradually with Peg-INF alpha plus ribavirin treatment. The differences were significant between day 0, 4th week and 12th week of treatment. The expression of sTRAIL was correlated with HCV RNA level at baseline, at 4th and 12th week of treatment (P = 0.021 P = 0.012, P = 0.001 respectively). IFN binds to its receptor on the infected hepatocyte surface during Peg-IFN alpha and ribavirin treatment. So the polarized phenotype of NK cell is not displayed and NK cell cytotoxicity such as TRAIL expression is blocked. We suggest that the decreased level of circulating sTRAIL may reflect increased binding to its ligand expressed on hepatocyte and decreased TRAIL production under the influence of Peg-IFN alpha plus ribavirin treatment. Therefore TRAIL may be probably a immunologically predictive factor such as HCV RNA during treatment.