In recent years, with the increasing heat dissipation of electrical appliances, efficient cooling systems are required to solve the elevated temperature problems encountered in certain interior areas of electrical devices. One solution is to use a vapor chamber heat spreader two-phase heat transfer device. A prototype vapor chamber heat spreader was built and tested for its thermal performance. The chamber had dimensions of 90 mm 90 mm 3.5 mm. It was sandwiched between a heater block and a water cooled plate. Heat was added to the bottom part of the vapor chamber heat spreader by a cartridge heater inserted in an aluminum block. The effective heating area was 12.7 mm 8.9 mm. The spreader upper surface was attached to an aluminum block, which was compressed against a copper water cold plate from which the heat was removed. The power input varied from 5 W to 50 W. Test results demonstrated that spreading resistance played a vital role in the total thermal resistance. The influence of inclination on the vapor chamber performance and temperature uniformity were also investigated.