This paper discusses the implementation of a 3 millimeter wave ( M M W ) linear frequency modulated (FM) source with high linearity for use in 3 MMW linear frequency modulated continuous wave ( F M C W ) radars. A linearization method is presented, which can perfectly achieve linear FM. Rased on this method ,a digital real-time closed-loop linearization system is developed, while integrated with a 3 MMW VCO, can make the maximum instantaneous linearity of the VCO be up to 0.016%.