Cardols are resorcinolic lipids, available in many natural sources including cashew nut, pistachio, macadamia, and mango. Despite of several beneficial biological activities of cardols, cytotoxic activities of cardols are not fully understood. In preliminary studies, 5[8(Z),11(Z),14-pentadecatrienyl]resorcinol, known as cardol (C15:3) was found to inhibit tyrosinase-catalyzed melanin formation in cell-free system. In the case of cultured cell analysis, cardol (C15:3) showed intense cytotoxicity but not anti-melanogenic activity against B16-F10 melanoma cells. Subsequently, cardol (C10:0) and cardol (C5:0), containing shorter alkyl side chain, exhibited inferior cytotoxicity compared to cardol (C15:3). The cytotoxicity via cardol (C15:3) was reversed by the addition of antioxidants, indicating that intracellular prooxidative activity was involved. Furthermore, cardol (C15:3) produced significant levels of reactive oxygen species (ROS) while cardol (C5:0) generated lesser ROS levels. Our findings suggest the cytotoxic activity of cardols is their prooxidative effect depending on the length of alkyl side chain.