Momentum Biased System is adopted as an attitude control system of Koreasat 12. Roll/pitch angles are measured by an Earth sensor, while other attitude data are not directly measured. Based upon kinematic coupling effect between momentum wheel and orbit rate, the other attitude data that are not measured can be estimated. In this research, nonlinear attitude dynamics of Koreasat 1,2 are derived and attitude estimation software using the Extended Kalman Filter are implemeted and tested by using the simulation data. In this way the attitude data not measured for a geosynchronous satellite using momentum biased system can be shown to be estimated in real-time.