MicroRNAs (miRNAs), a class of small, single-stranded endogenous RNAs, act as post-transcriptional regulators of gene expression. The ability of one single miRNA regulating multiple functionally related mRNAs makes it a new potential candidate for cancer gene therapy. Let-7s miRNAs have been demonstrated as tumor-suppressor genes in various types of cancers, providing one choice of gene therapy by replenishing this miRNA. In the present studies, we demonstrate that the chemically synthesized, double-stranded Let-7 mimics can inhibit the growth and migration and induce the cell cycle arrest of lung cancer cell lines in vitro. Let-7 mimics silence gene expression by binding to the 3' UTR of targeting mRNAs. Mutation of seed sequence significantly depresses the gene silencing activity of Let-7 mimics. Our results also demonstrate that it is possible to increase the activity of Let-7s through mutating the sequence within the 3'end of the antisense strand. Directly, co-transfection Let-7 mimics with active siRNAs impairs the anti-cancer activities of Let-7 mimics. However, a 3-h interval between the introduction of Let-7 mimics and a kind of siRNA avoids the competition and enhances the anti-cancer activities of Let-7 mimics. Taken together, these results have revealed that Let-7s mimics are potential candidates for cancer gene therapy.