Based on the principle and method for calculating probability of causation in current National Standard, we developed a computer program on probability of causation calculation for cancer patients caused by ionizing radiation with Visual Studio 2008. We were guided by Interactive Radio-Epidemiology Program developed by National Institutes of Health (NIH-IREP) and National Institute for occupational Safety and Health (NIOSH-IREP). Computer calculating system for probability of causation for radiogenic cancers and its users’ guide were finished. This computer program had undertook calculation accuracy testing for three times to guarantee its accuracy of calculation. And it greatly improves operability of calculation for probability of causation.Cited