Osteosarcoma, the most common primary bone tumor in young adults, is characterized by local invasion and distant metastasis. But detailed mechanisms of tumorigenicity and metastasis of osteosarcoma are not well known. We report the involvement of calpains, a family of calcium-activated, cysteine proteases, in the invasive and metastatic processes of human osteosarcoma cells. By using siRNA treatment, the expression of mu- and m-calpains were downregulated in human Saos-2 osteosarcoma cells. Both the adhesive and invasive potentials were significantly attenuated in calpain siRNA-transfected human Saos-2 osteosarcoma cells. MMPs are the main factors involved in malignant tumor invasion and metastasis. siRNA of calpains also significantly inhibited the secretion of MMP-2 in Saos-2 cells. These results suggest that mu- and m-calpains are important in the invasion and metastasis of human osteosarcoma cells, and calpains might be targeted to reduce tumor progression.