A new frequency modulation system for lasers by heterodyne bias frequency-locking is proposed and developed. The technique can be used in heterodyne or coherent single mode optical communication. Experiments on grating-tuned CO2 laser system and Zeeman He-Ne laser system are described. Radio broadcast has been relayed in the locked system successfully. The conditions for practical optical fiber communication with semiconductor laser of heterodyne bias frequency-locked system are discussed.