MicroRNAs (miRNAs) are small noncoding RNAs comprising 21-23 nucleotides that regulate gene expression by transcriptionally repressing their complementary mRNAs. In particular, let-7 miRNA has been postulated to function as a tumor suppressor in various cancer cells, but not yet in glioblastoma. In this study, we investigated the anti-tumorigenic effect of let-7 miRNA in glioblastoma cells. Human glioblastoma cells (U251 or U87 cells) were transfected with let-7 miRNA and assayed for in-vitro proliferation, migration, and in-vivo tumor formation. Transfection of let-7 miRNA reduced expression of pan-RAS, N-RAS, and K-RAS in the glioblastoma cells. Let-7 miRNA also reduced the in-vitro proliferation and migration of the cells, and reduced the sizes of the tumors produced after transplantation into nude mice. However, let-7 miRNA exerted no effect on the proliferation of normal human astrocytes. These results indicate that let-7 miRNA has an anti-tumorigenic effect on glioblastoma cells, and suggest possible use of let-7 miRNA for treating glioblastoma.