Second mitochondria-derived activator of caspase (Smac) is a mitochondrial protein that promotes apoptosis in many kinds of cancers. Here, for the first time, the effects of Smac RNAi on growth and drug resistance to cisplatin [cis-diamminedichloroplatinum(II)] of lung cancer cells are investigated. Knockdown of Smac expression in A549 and 95D cells was mediated by transfection with pGC-FU vector containing siRNA sequences targeting human Smac with the lentivirus vector system. Smac was also overexpressed by transfection with pOE vector containing full-length coding region of Smac. Cell growth, cell cycle, and apoptosis were measured by methyl-thiazol tetrazolium assay, colony-formation assay, and flow cytometry. Drug resistance was performed by treatment with 10 μg/mL cisplatin. Downregulation of Smac enhanced cell growth and drug resistance to cisplatin of A549 and 95D cells, whereas overexpression of Smac did reversely. Smac helps inhibit cell growth and potentiate drug sensitivity to cisplatin of lung cancer cells.