In the present study, the operating limit of micro pulsating heat pipes (MPHPs) was investigated with different channel geometries: cross-sectional shapes and hydraulic diameters. Five-turn closed-loop MPHPs, which have either circular or square channels with hydraulic diameters of 390, 480, and 570 gm, were fabricated onto a silicon wafer using MEMS techniques. To allow flow visualization, a transparent glass wafer was bonded to the silicon wafer. FC-72 was used as the working fluid with a filling ratio of 50%. Experimental results show that the operating limit, where the thermal performance significantly deteriorates, increases with the hydraulic diameter in both cross-sectional shapes. At the same hydraulic diameter, the square-channel MPHP can handle approximately 70% higher maximum allowable heat flux than the circular-channel MPHP. Based on the results of flow visualization, a model for the operating limit of the MPHPs in a vertical orientation was proposed: It was postulated that the MPHPs reach the operating limit when the falling film flow rate is smaller than the evaporation rate at any instant the liquid film is formed. This postulate was experimentally confirmed by comparing two rates for various channel geometries. Finally, a correlation for predicting the maximum allowable heat flux of the MPHPs was developed using a scale analysis and experimental data. According to the model, the maximum allowable heat flux is found to be proportional to the third power of the hydraulic diameter regardless of the cross-sectional shape. (C) 2016 Elsevier Ltd. All rights reserved.