Apoptosis is an active form of cell death that is initiated by a variety of stimuli, including reactive oxygen species (ROS) and ultraviolet (UV) radiation. Results in this study demonstrated that 1) Poly(ADP-ribose) (PAR) is a new marker of UVB- and H2O2-induced HaCaT cell apoptosis. 2) UVB and H2O2 induce HaCaT cell apoptosis in different pathways, DNA damage in the form of cyclobutane thymine dimmer formation may play a major role in UVB-induced apoptosis. Furthermore, ERK signaling pathway is not involved in UVB-induced HaCaT cell apoptosis.