In this paper,a novel demodulation algorithm without constellation knowledge ( CK) for Amplitude-Phase Keying ( APK) signals is introduced,which is based on the minimum probability of symbol error ( MPSE) criterion,optimal in performance and suitable for non-cooperative communication. For reducing the complexity,an on-line algorithm is derived. Using preceding algorithms,asymptotic performance of the proposed algorithm is proved theoretically. Numerical simulations are also provided to validate the analytical results. Our study of asymptotic performance shows that the performance of demodulation for any APK signals without CK is identical with the APK optimal receiver with CK when the number of available data symbols goes to infinity. In this paper, a novel demodulation algorithm without constellation knowledge (CK) for Amplitude-Phase Keying (APK) signals is introduced, which is based on the minimum probability of symbol error (MPSE) criterion, optimal in performance and suitable for non-cooperative For reducing the complexity, an on-line algorithm is derived. Using preceding algorithms, asymptotic performance of the proposed algorithm is proven theoretically. Our study of asymptotic performance shows that the performance of demodulation for any APK signals without CK is identical with the APK optimal receiver with CK when the number of available data symbols goes to infinity.