In passive millimeter-wave imaging systems used indoors, the radiometric temperature contrast is barely enough for coarse object detection, being usually insufficient for recognition due to the absence of cold sky. The image contrast results from a combination of emissivity and reflectivity which are dependent on the dielectric constant of objects, the angle of incidence, and the polarization direction. To improve the capability of target recognition, we proposed the linear polarization sum imaging method which is based on the combination of the different polarization images for increasing the intensity contrast between the target area and the background area. In order to capture the linear polarization sum images of a metal sphere, a metal and a ceramic cup, we designed W-band quasi-optical imaging system which can generate the polarization dependent images by manually changing the linear polarization direction of its radiometer receiver from 0 to π /2 by the step size of π/8. The theoretical and experimental results of the linear polarization sum imaging show that it is capable for achieving good image quality enough to recognize the target.