Purpose: To investigate alterations of mitochondria in irradiated endothelial cells to further elucidate the mechanism underlying radiation-induced heart disease.Materials and methods: Experiments were performed using human umbilical vein endothelial cells (HUVECs). HUVECs were irradiated with single gamma ray dose of 0, 5, 10 and 20Gy, respectively. Apoptosis was assessed by flow cytometry at 24, 48 and 72h post-irradiation, respectively. The intracellular reactive oxygen species (ROS) was measured with 2,7-dichlorofluorescein-diacetate (DCFH-DA) at 24h post-irradiation. Mitochondrial membrane potential (m) by JC-1 and the opening of mitochondrial permeability transition pore (mPTP) by a calcein-cobalt quenching method were detected at 24h post-irradiation in order to measure changes of mitochondria induced by gamma ray irradiation.Results: Gamma ray irradiation increased HUVECs apoptosis in a dose-dependent and time-dependent manner. Irradiation also promoted ROS production in HUVECs in a dose-dependent manner. At 24h post-irradiation, the results showed that irradiation decreases m, however, paradoxically, flow cytometry showed green fluorescence instensity higher in irradiated HUVECs than in control HUVECs in an irradiation dose-dependent manner which indicated gamma ray irradiation inhibited mPTP opening in HUVECs.Conclusions: Gamma ray irradiation induces apoptosis and ROS production of endothelial cells, and decreases m meanwhile contradictorily inhibiting the opening of mPTP.