The tumor suppressor, p53 is a short-lived nuclear phosphoprotein that functions as a DNA damage-inducible sequence specific transcription factor. The level of p53 protein depends on the UVC dosages during UVC-induced apoptosis in SK-HEPl cells. p53 protein exists on the high level at lower UVC dosages, but p53 protein exists on the low level at higher UVC dosages. The stability of p53 protein rapidly increases with lower UVC dosages, but the stability of p53 protein slowly increases with higher UVC dosages. The stability of p53 protein is regulated by Mdm2/Proteasome degradation pathway. The regulation does not seem to be attributable to the change of proteasome activity. Increased Mdm2 expression in cells received high intensity of UVC suggests that the regulation might be controlled by Mdm2-mediated ubiquitination. The mechanism regulating the stability of p53 protein during UVC-induced apoptosis in SK-HEPl cells was discussed.