In this paper, an analysis of an electric power system with dc transmission is performed based on a differential-algebraic equation (DA equation). The DA equation is derived to represent the dynamics of the generator and the dc transmission, and the instantaneous real and imaginary power relationship between the ac system and the dc transmission. The DA equation is regarded as a mathematical model for analyzing transient stability of the ac/dc system. Through the DA equation, the dynamic behavior related to the transient stability is numerically discussed.