Experiments were performed to investigate the effect of a dual-diameter channel on the flow and heat transfer characteristics of flat plate micro pulsating heat pipes (MPHPs). Using MEMS techniques, the MPHPs were fabricated: rectangular channels with dual hydraulic diameters were engraved on a silicon wafer to form a meandering closed loop with 5 turns. The Pyrex glass was used as a cover which enables flow visualization in the MPHP. Experimental results show that the MPHPs with a dual-diameter channel can operate stably even in a horizontal position and have uniform thermal performance regardless of the orientation when a proper condition is met. It is postulated that the orientation-independent performance occurs when the capillary pressure generated by the difference in channel diameters is larger than the viscous pressure drop. This postulate was experimentally confirmed using Ethanol and FC-72 as working fluids at various input powers and inclination angles. To provide a guideline for orientation-independent operation of the MPHP with a dual-diameter channel a figure of merit is proposed: the figure of merit is defined as the ratio of capillary pressure difference to the viscous pressure drop, which is similar to the figure of merit for a conventional heat pipe. According to experimental data, the figure of merit has to be larger than 2 x 10(5) for orientation-independent operation of the MPHP. (C) 2015 Elsevier Ltd. All rights reserved.