In this paper, a new Open and Closed Loop Alternation(OCLA) frequency synthesizer is developed to eliminate a frequency error occurring in the transition state of a frequency hopping. This frequency synthesizer consists of a phase comparator(PC), two low pass filters(LPF), two voltage controlled oscillators(VCO), switching elements, a programmable divider and frequency hopping controller, and the stabilized output frequency can be obtained by switching performance. In addigion, it can be found that the characteristic of its circuit construction makes it easy to attach an external circuitry to the open loop.