This paper develops a differential encoder for differentially demodulated continuous phase frequency shift keying (CPFSK). CPFSK schemes with modulation index , where and are relatively prime positive integers, can be represented by a decomposed model consisting of a continuous phase encoder (CPE) and a memoryless modulator (MM). The differential encoder is shown to fit well with the CPE and form a decomposed model of differentially encoded CPFSK (DCPFSK). A basic receiver structure for differentially demodulating DCPFSK is presented along with simulation results. An exact formula for the minimum squared Euclidean distance (MSED) of differentially demodulated DCPFSK is also given.