Quantitative determination of trans-cinnamaldehyde (TCA) was conducted by reversed- phase HPLC from young and mature leaves, and leaf branches of Cinnamomum osmophloeum, a Taiwan endemic plant. The results showed that highest yield, 23.79 mg/g of TCA (the tree’s age was three years) was obtained in the two year old mature leaves. In addition, cytotoxic and inhibitory effects of TCA was evaluated against selected human cancer cell lines such as Jurkat, U937, and normal cell lines primary purified T cells and macrophages. The results revealed that TCA exhibited potent inhibitory activity against Jurkat and U937 cell viability, and found that the IC50 values were 0.057, 0.076 μM, respectively. In parallel, no effect on the viability of primary purified T cells and macrophages. Moreover, interestingly at 0.095 μM, TCA inhibited proliferation of both Jurkat and U937 cell lines approximately 2-fold at 0.057 μM, compared to controls. In contrast, TCA increases approximately 26% proliferation of mitogen- stimulated human peripheral blood mononuclear cells (PBMCs) during the concentration range studied. Furthermore, by cell cycle analysis, we found that TCA altered the cell cycle phase distribution of Jurkat and U937 cells in a nonlinear concentration-dependent fashion. Taken together our results suggest that TCA may be a useful chemotherapeutic agent for cancer treatment in human.