The flat plate solar water heater, using the closed-loop oscillating heat pipes, was constructed and investigated. The flat plate collector consisted of 10 pipes of closed-loop oscillating heat pipes and the collector area was 1.5?1 m2. Each closed-loop oscillating heat pipes was made of a copper capillary tube with a 1.5 mm inner diameter, a 2.8 mm outer diameter, and had 20 turns. The distilled water was used as the working fluid with a filling ratio of 50% the tube?s total internal volume. The evaporator section of the closed-loop oscillating heat pipes was placed on the absorber plate of the collector, and its condenser section was wrapped around the copper tube, in which hot water flowed through. The solar water heater was tested under the solar simulator with halogen lamps generating the uniform artificial solar energy. The irradiation intensity and the water flow rate of the solar water heater were adjusted. It was found that the thermal performance of the solar water heater clearly improved with an increase in the irradiation intensity from 480-1086 W/m2. However, the water flow rate in the range of 1.5-3.0 Lpm, had a thermal performance that was slightly different. The thermal efficiency of 0.67 was archived at the high irradiation intensity of 947-1086 W/m2. Moreover, the mathematical model to predict the thermal efficiency of the flat plate solar water heater with the closed-loop oscillating heat pipes was obtained.