The thermal performance of a flat-plate oscillating heat pipe operating as a heat spreader was experimentally investigated. The copper flat-plate oscillating heat pipe had overall dimensions of 101.6 × 101.6 × 3.18 mm3, possessed two layers of 1.02 × 1.02 mm2 minichannels, and was charged with acetone to a filling ratio of approximately 80%. The flat-plate oscillating heat pipe was centrally heated witha9 cm2 square heat source and had a copper pin-fin heat sink attachedtoits opposite side for crossflowing airatspeedsofeither1,2, or 3 m/s. The results clearly indicated that the flat-plate oscillating heat pipe is capable of operating in the thermal spreading mode, providing fora10-15% reductionintotal thermal resistance (relativeto pure copper) and managing heat inputsupto 230 W (∼25 W/cm2) with the maximum surface temperature not exceeding 100°C. In addition, the temperature uniformity of the heat sink was greater when attached to the flat-plate oscillating heat pipe, especially for lower Reynolds numbers (airspeeds). A critical heat input was requiredtoinitiate the flat-plate oscillating heat pipe, and this heat input increased with Reynolds number. This study further confirms the heating mode directly affects the heat transfer performance of a flat-plate oscillating heat pipe. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.