In this paper, effect of gradually expanding flow passages on saturated flow boiling characteristics of micro pin fin heat sinks is experimentally investigated. In this regard, a comparative analysis is performed based on three different heat sinks having straight parallel channels, uniformly distributed micro pin fins, and decreasing number of micro pin fins in flow direction. For these three geometries, visualization supported experiments are conducted, approximately, at the mass flux of 212 kg m(-2) s(-1), inlet temperature of 75 +/- 1 degrees C and effective heat fluxes of 132 - 272 kW m(-2). Working fluid was deionized water. It is concluded that gradually expanding flow passages towards the outlet significantly improves the two-phase heat transfer coefficient up to nearly 501% and 365% compared to straight parallel channels and uniformly distributed micro pin fins, respectively. The structure having decreasing number of micro pin fins obviously suppresses flow boiling instabilities, and also presents lower pressure drop (lower up to 1.45 times) than uniformly distributed micro pin fins. Gradually enlarging flow passages provide escape-ways in the highly saturated region; thus, accelerate the flow and shorten the waiting period of vapor on heat transfer surface. Important conclusions are obtained regarding flow mechanism via flow images. (C) 2022 Elsevier Ltd. All rights reserved.