This research aimed to investigate the antioxidant effect from rhizomes of K. rotunda for finding the active compounds by DPPH free radical scavenging activity assay. The chloroform-soluble extract of the rhizomes of K. rotunda showed significant scavenging effect on the on 1,1-diphenyl-2-picryl hydrazyl (DPPH) free radicals (IC50 = 180 microg mL(-1)). Two compounds of the chloroform-soluble extract were isolated and identified. Compound 1, 2'-hydroxy-4,4',6'-trimethoxy-chalcone was found as the active constituent (IC50 = 142 microg mL(-1)). Compound 2, (+)-crotepoxide, was inactive (IC50 = 1516 microg mL(-1)). The structures of compounds 1 and 2 were identified based on the basis of spectral evidence, Mass Spectrophotometry (MS) and 2D-NMR (2 dimension of Nuclear Magnetic Resonance) data including Heteromolecular Multiple Quantum Coherence (HMQC) and Heteromolecular Multiple Bond Correlation (HMBC) and comparison to published values.