On the basis of studying the principle of passive millimeter wave (PMMW) imaging, a compact 3mm super-heterodyne Dicke-radiometer was developed and the imaging experiment was done. An improved maximum-likelihood (IML) super-resolution algorithm was proposed in order to solve the inherent problem of poor resolution imaging, which was caused by limited aperture dimensions and the consequent diffraction limit. Experiment results indicate that the algorithm is able to complete spectral extrapolation, and enhance super-resolution performance and spatial resolution. The primary parameters of the 3mm band radiometer are given as follow： the center frequency is 91.5 GHz, the system band-width is 4GHz, the sensitivity of temperature is 0.55K and the linearity is 0.9999.