A clock generator is illustrated. The clock generator mentioned above includes a multimodulus frequency divider and a delta-sigma modulator. The multimodulus frequency divider is archived by switching the phase thereof. The multimodulus frequency divider increases the operating frequency of the clock generator effectively, and has a characteristic with half period resolution for reducing the jitter of an output clock signal when its spectrum is spread. Besides, the delta-sigma modulator increases the accuracy of the triangle modulation and reduces error of quantization by adding a few components therein. Thus, the clock generator could be expanded to a programmable clock generator.