In this study, we investigated the process of X-ray-induced apoptosis of skin keratinocyte, and the functional role of protein kinase C delta (PKCdelta) and downstream signalling cascade. High-dose X-ray irradiation (10 Gy) led to the apoptosis of HaCaT keratinocyte, accompanied by PKCdelta cleavage. Treatment with PKCdelta inhibitor and adenoviral transduction of dominant-negative PKCdelta clearly inhibited the X-ray-induced apoptosis of keratinocyte. In addition, X-ray induced the phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2) and inhibition by ERK1/2 inhibitor abrogated the X-ray-induced apoptosis. Interestingly, overexpression of dominant-negative PKCdelta markedly blocked the X-ray-induced phosphorylation of ERK1/2, suggesting that ERK1/2 is the functional downstream effector of PKCdelta. Next, we investigated the difference between UVB and X-ray response. UVB induced the apoptosis of keratinocyte in a PKCdelta-dependent manner, similar to X-ray response. However, UVB irradiation induced the phosphorylation of c-jun N-terminal kinases (JNK) and inhibition of JNK significantly protected the UVB-induced apoptosis. These results demonstrate that PKCdelta is a key regulator in X-ray-induced apoptosis of keratinocyte and suggest that there is subtle difference in downstream signalling cascade between UVB and X-ray response of keratinocyte.