Optical feedback characteristics of frequency modulated microchip Nd：YAG lasers are investigated. Without optical feedback，the output intensity changes by one fringe with the laser frequency changing by one longitudinal mode spacing. With optical feedback，the modulation frequency of output intensity is proportional to the ratio of external cavity length to internal cavity length. The theoretical analysis based on three-cavity-mirror equivalent model is in good agreement with experimental results. The potential applications of this phenomenon in precision measurement are also discussed.