The bioassay-guided fractionation of the CHCl 3 extract from pigeon pea leaves led to the isolation of a new natural coumarin, named as cajanuslactone ( 1), together with two known phytoalexins, pinostrobin ( 2) and cajaninstilbene acid ( 3). The structure of 1 was elucidated by interpreting spectroscopic data of UV, IR, MS, 1D and 2D NMR. It was determined as 7-hydroxy-5- O-methyl-8-(3-methyl-2-butylene)-4-phenyl-9,10-dihydro-benzopyran-2-one. The anti-bacterial activity against three Gram-positive microorganisms was evaluated in vitro. It was found the new compound cajanuslactone possessed good anti-bacterial activity against Staphylococcus aureus (ATCC 6538), and the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) were 0.031 and 0.125 mg/ml, respectively. The result indicated that the leaf part of pigeon pea is an excellent source of natural anti-bacterial materials. Moreover, cajanuslactone is a potential anti-bacterial agent against Gram-positive microorganisms.