A new coumarin derivative, coumacasia (1) and eight known compounds, coumarin (2), cinnamaldehyde (3), 2-methoxycinnamaldehyde (4), 2-hydroxycinnamaldehyde (5), coniferaldehyde (6), cinnamic acid (7), 2-hydroxycinnamic acid (8), and cinnamic alcohol (9), were isolated from the methanol extract of Cinnamomum cassia. Their structures were elucidated by spectral data and by comparison with the reported literature. The cytotoxic activities of compounds 1-9 were evaluated with two human cancer cell lines, HL-60 and A-549. Compound 1 showed growth inhibitory effects in the HL-60 and A-549 cell lines with IC50 values of 8.2 +/- 0.5 and 11.3 +/- 1.1 microM, respectively. Compounds 3-6, and 8 exhibited moderate cytotoxicity with IC50 values ranging from 20.5 to 65.6 microM.