Recently, ITS (Intelligent Transport System) has been studied actively which is useful for smooth traffic and safety drive of the various vehicles. Though the laser or imaging camera sensor was used for ITS, they have unstable performances and high complexity. Therefore, FMCW radar using millimeter wave has been proposed recently. Actually, 47, 60, 77, 94 and 139 GHz are assigned for the vehicle radar frequency in Europe and Japan. In this paper, FMCW radar was simulated and modeled with interference. Also, RER(Range Error Rate) was proposed to estimate the performance error due to the interference.