Docosahexaenoeic acid (DHA, 22:6 n-3) is an omega-3 polyunsaturated fatty acid that is found in fish oil and exerts cytotoxic effect on a variety of cell lines. The molecular target, responsible for mediating this effect of DHA, still remains unknown. In this report, we presented experimental evidences for the role of PPAR-gamma in conveying the cytotoxic effect of DHA. We showed that DHA induces apoptosis in Reh and Ramos cells and apoptotic effect of DHA is inhibited by the PPAR-gamma antagonist GW9662, indicating that PPAR-gamma functions as the mediator of the apoptotic effect of DHA. Furthermore, our result showed that DHA induces the PPAR-gamma protein levels in both Reh and Ramos cells. Interestingly, DHA was found to induce the expression of p53 protein in Reh cells in a PPAR-gamma-dependent manner. The up-regulation of p53 protein by DHA kinetically correlated with the activation of caspase 9, caspase 3, and induction of apoptosis, suggesting a role for p53 in DHA-mediated apoptosis in Reh cells. Taken together, these findings suggest a new signaling pathway, DHA-PPAR-gamma-p53, in mediating the apoptotic effect of DHA in Reh cells.