Abstract: The reason for "part action with full responsibilities" in joint principal offenders is that the offenders are related to the consequences both mentally and physically, which is the nature of joint principal offense. The joint behavior theory ignores the act intention, a subjective element, and it cannot distinguish offense at the same time and offense joint together. At the same time, the joint crime theory wastes time on finding a concrete crime name that the offenders committed, which is insignificant. Therfore neither the joint behavior theory nor the joint crime theory can interpret the principal of "part action with full responsibilities" in joint principal offenders, and these two theory should be abandoned.