In discussion on reduction of energy and an environmental issue on a global scale, the research project had been organized in Japan. In the project, a committee “Development of model for the high accurate prediction of welding distortion by computer simulation” was set up for the purpose of reduction of energy spent for correction of welding distortion. A lot of items were investigated. The research was carried out to find the dominant factor for the high accurate prediction of welding distortion generated by fillet welding in the committee. As the results, two dominant factors were found and welding distortion generated by fillet welding could be predicted with high accuracy by satisfying two dominant factors.