An alkaloid, 3-methylcanthin-5, 6-dione, was isolated from the stem of Picrasma quassioides (D. Don) Benn. and characterized by comprehensive analyses of its 1D and 2D NMR spectra. It was also evaluated for its cytotoxic activity in vitro against three human cancer cell lines (MDA-MB-231, HT-29 and NCI-N87), using MTT assays. We found that 3-methylcanthin-5, 6-dione exhibited significant anti-inflammatory activity via inhibiting NO production induced in LPS-stimulated murine macrophage RAW264.7 cells. The antioxidant activity of 3-methylcanthin-5, 6-dione was measured by DPPH free radical scavenging assays, hydroxyl radical scavenging assays and reducing power assays. Our results showed that 3-methylcanthin-5, 6-dione has significant biological activities.