In this paper, we develop and analyze optimum asynchronous detector for M-ary Frequency Shift Keying (MFSK) signals traveled over Rayleigh fading channel. Developing this detector comprises two steps: (1) deriving a synchronous optimal detector based on applying the statistical decision theory to the Rayleigh fading MFSK signal, (2) modifying the synchronous structure of the MFSK detector by averaging over the unknown signal arrival time. The performance of the asynchronous detector is measured in terms of the probability of mis-detection and is compared with the performance of the synchronous one. The asynchronous detector operates satisfactory under lack of knowledge of the arrival time and its performance enhances by increasing the quantization level of epoch.