Millimeter wave characteristics differ from those of microwaves, infrared or optical system, and it has some distinct advantages over microwave, infrared, or optical system. Nowadays, precision guided weapons or end-sensing-cartridges are often two active and passive modes of operation for a millimeter wave seeker. In view of the characteristics of the coating-stealth objections, we deal with a new method for anti-coating-stealth objective by passive millimeter wave seeker system. The principle of coating-stealth targets to be detected with passive millimeter wave seeker was analyzed. Then, according to data of antenna temperatures resulted from the coating stealth targets, a method for calculating the minimum variance and the best value of the coating-stealth target’s radiation temperatures with passive millimeter wave seeker was introduced. Moreover, the experimental data with 3mm passive radiometer were analyzed. Finally, simulation learning curve was given.