Studies have shown that miR-145-3p functions as a tumor suppressor and is associated with tumor growth and metastasis. This study intends to uncover the mechanism of a tumor suppressor of miR-145-3p. The expressions of miR-194 in osteosarcoma cell lines and tissues were monitored by real-time PCR. The proliferation ability was examined by MTT assay. The apoptosis and autophagy of cells were monitored by flow cytometry and microcopy， respectively. The regulation of miR-145-3p on HDAC4 was determined by luciferase assays and western blot assay. The results showed that miR-145-3p was significantly reduced in the osteosarcoma compared with the normal bone tissue. Overexpression of miR-145-3p significantly attenuated the proliferation and induced the apoptosis and autophagy of osteosarcoma cells. Furthermore， we demonstrated that miR-145-3p has inhibited the malignant behavior of osteosarcoma by down-regulating HDAC4 expression. These findings suggested that miR-145-3p may act as a tumor suppressor in osteosarcoma. MiR-145-3p/HDAC4 may be a novel therapeutic target in treatment of osteosarcoma.