This paper presents a new computation method for determining critical clearing time (CCT) on the transient stability of power system. The critical trajectory is used as the boundary of stability in the CCT determination. This method also uses a modified trapezoidal equation for numerical integration process. Addition feature, simultaneous equations in the Jacobi's matrix will reduce the equation, so that the number of equations equal to the number of variables. This technique will reduce the simulation time for numerical integration process. The results show that the calculation time of proposed method is faster than the previous method. Therefore it is potentially for on line transient stability assessment.