Abstract: Objective: To investigate the effect and mechanism of all-trans-retinoic acid(ATRA) in inducing apoptosis of human small cell lung cancer(SCLC) cell line SH-77.Methods: SCLC cell line SH-77 was treated with different concentrations of ATRA.The multiparameter such as cell growth inhibition rate,proliferation index(PI),apoptosis index(AI) and Bcl-2 protein expression was detected by Typan blue stainning,MTT and flow cytometry,respectively.Results: Different concentrations of ATRA could inhibit cell line SH-77 cell growth.G₁/G₀ stage and AI were increased,while cell count at S stage and PI were reduced.Bcl-2 protein expression was decreased (P<0.01).Conclusions: ATRA can inhibit SCLC cell lines SH-77 growth,induce cell apoptosis and reduce Bcl-2 protein expression.