To evaluate cytotoxic effect and topoisomerase I inhibition activity of Caesalpinia sappan L., both water and methanol extracts were examined using in vitro assay. The cytotoxic effect of Caesalpinia sappan L. examined using MTT and SRB assay and $IC_{50}$ values were measured against U937, HL60, HepG2, SNU-1, SNU-16 cancer cell lines. Among them the representative cytotoxic results are shown as follows; water extract (U937=13.39 ${mu}g/ml$, HL60=8.65 ${mu}g/ml$, HepG2=38.48 ${mu}g/ml$, SNU-1=7.72 ${mu}g/ml$, SNU-16=25.49 ${mu}g/ml$), methanol extract (U937=13.35 ${mu}g/ml$, HL60=9.43 ${mu}g/ml$, HepG2=25.67 ${mu}g/ml$, SNU-1=8.37 ${mu}g/ml$, SNU-16=28.64 ${mu}g/ml$). The inhibitory concentration of DNA topoisomerase I activity against water extract was 100 ${mu}g/ml$ and the inhibitory concentration of DNA topoisomerase I against methanol extract was 400 ${mu}g/ml$.