This paper describes an optimal method of evaluating the synchronous reactance of the d- and q-axes simultaneously with rotor angle measurement. First, interactions among the variables such as the synchronous reactance of the d- and q-axes, the field current and the rotor angle through magnetic saturation are theoretically reviewed with a standard machine model. Next, with the steady-state test data, optimal values of X-d and X-q are evaluated simultaneously using an optimization technique to minimize the sum of 2-norm errors between the calculated and measured values in the field current and the rotor angle. The proposed method has been verified against both the 282 MVA salient-pole and 612 MVA round-rotor generators. The optimal value of the X-q/X-d ratio was found to be much smaller than that of manufacturer's data, and mismatches between the measured and calculated values in both the rotor angle and field current were significantly reduced. This systematic method is so simple that field engineers would find it easy to use. Thus, the rotor angle measurement makes it possible to derive the optimal values of X-d and X-q simultaneously, and the use of high-precision speed pickup sensor from all the generators is essential for improving machine model accuracy.