A useful method is presented for the evaluation of the error probability ofM-ary phase-shift-keyed systems with Gaussian noise and intersymbol interference. By regarding a portion of the intersymbol interference as an equivalent Gaussian noise, this method suggests first calculating the probability of error caused by the combined effects of the Gaussian noise and the equivalent noise. Necessary correction terms are then added to the calculation to take into account the fact that the intersymbol interference is actually non-Gaussian. It is shown that with this method any desired accuracy can be theoretically achieved in the evaluation of the probability of error. This method has been programmed for numerical computations. Some examples are also given for illustrative purposes.