Pinus massoniana bark extract (PMBE) is a mixture of flavonoids, and showed a capability of inducing cell apoptosis; however, its properties have not yet been fully investigated. This paper evaluates the antitumor effects of PMBE in murine sarcoma S180 both in vitro and in vivo. In vitro, the growth inhibition of S180 cells was concentration dependent on PMBE as shown by the CCK-8 assay. The AO/EB staining and flow cytometry assay showed that PMBE induced S180 cell apoptosis. Cell cycle analysis revealed that the cells in the S phase were decreased by treatment with PMBE. In vivo, the treatment of 100, 200, and 300 mg/kg PMBE reduced the tumor weight and volume of S180-bearing NIH mice by 9%-67% and 13%-68%, respectively. Peripheral leukocyte count and lymphoproliferation were increased significantly after treatment with PMBE. Our results suggest that PMBE inhibits the tumor cell growth by inducing cell apoptosis and improving lymphoproliferation.