Abstract: The copper-based flat-plate oscillating heat pipes（FPOHPs） with thickness of 1.00 mm were designed and fabricated by printed circuit technology. The dual-diameter channels in the FPOHP with two neighboring channel widths of 1.00 and 2.20 mm were arranged, respectively. At the bottom of each wider channel, the parallel microgroove structures were respectively wet-etched. HFE-7100 was used as working fluid, and the start-up and the heat transfer performance of ultra-thin FPOHPs with/without micro grooves were compared at different filling ratios ranging from 30% to 50%. The experimental results show that the FPOHPs with/without micro grooves can start up and operate stably at both horizontal and vertical orientations, and the addition of microgroove structures can significantly enhance the FPOHP performance and lower the evaporator temperatures. The gravity independence of the FPOHP is improved after using microgroove structures. For the power input of 24 W, the effective thermal conductivities of the micro-grooved FPOHP at 30% filling ratio are respective 1 241 and 1 226 W/（m·℃） at horizontal and vertical orientations, which are increased by 25.6% and 18.7% as compared with those without micro grooves. The ultra-thin printed circuit FPOHP can largely meet the demand of electronic chip cooling.