The effects of window cover on the performance of the plate type radiant cooling test system are studied. The cover materials tested are 2mm, 1mm, 0.7mm glass, Cokin 007 filter, and PVC film. The measurements are performed during the September nights of 2005 at the rooftop of the building in Seoul. The equivalent height of the building is about a building with twelve floors. The radiant cooling performance of the black painted and aluminum film coated plate are compared under no cover condition. Also the results are compared when the system has cover over the plate. In general, the covers reduce the radiant cooling temperature differences during the nighttime. Among these covers, the PVC film cover shows the best performance. In some cases, the plate with PVC film cover shows lower temperatures than the system without cover.