miR-15a and miR-16-1 downregulate CCND1 and induce apoptosis and cell cycle arrest in osteosarcoma. Osteosarcoma, the most common primary tumor of the bones, causes many deaths due to its rapid proliferation and drug resistance. Recent studies have shown that cyclin D1 plays a key regulatory role during cell proliferation, and non-coding microRNAs (miRNAs) act as crucial modulators of cyclin D1 (CCND1). The aim of the current study was to determine the role of miRNAs in controlling CCND1 expression and inducing cell apoptosis. CCND1 has been found to be a target of miR-15a and miR-16-1 through analysis of complementary sequences between microRNAs and CCND1 mRNA. The upregulation of miR-15a and miR-16-1 in the cell line SOSP-9607 induces apoptosis and cell cycle arrest. Osteosarcoma cells transfected with miR-15a and miR-16-1 show slower proliferation curves. Moreover, the transcription of CCND1 is suppressed by miR-15a and miR-16-1 via direct binding to the CCND1 3'-untranslated region (3'-UTR). The data presented here demonstrate that the CCND1 contributes to osteosarcoma cell proliferation, suggesting that repression of CCND1 by miR-15a and miR-16-1 could be used for osteosarcoma therapy.