Polyalthia is a versatile genus of shrubs and trees found in tropic and sub-tropic regions. In this study, three clerodane diterpenes, kolavenic acid (1), polyalthialdoic acid (2), and 16α-hydroxy-cleroda-3,13(14)Z-dien-15,16-olide (3) isolated from Polyalthia longifolia leaves were evaluated for their apoptotic potential against human leukemia HL-60 cells. Compounds 2 and 3 inhibited cell proliferation with IC₅₀ values of 21.8 and 13.7 μM, respectively. Morphological changes and DNA fragmentation analysis indicated that these diterpenes induce apoptotic cell death in the HL-60 cells. Our results revealed the importance of P. longifolia as a chemopreventive medicinal plant.