Experiments on flow boiling were conducted in stepped fin microchannels. The test vehicles were made from copper with two different footprint areas, designated as SFMC1 and SFMC2. The microchannels were formed by wire cut Electro Discharge Machining process and have surface roughness (Ra) of about 2.0 mu m. Tests were performed, with deionized water, on channels having nominal width of 300 mu m and a nominal aspect ratio of four over different mass velocity and heat flux range and inlet temperature of 90 degrees C. The experimental results of flow boiling in stepped fin microchannels and discussion on its heat transfer characteristics, instabilities and two-phase pressure drop are presented. The experimental data was plotted on Taitel-Dukler flow regime map to predict the flow boiling regimes existing within the channel under different operating conditions. The heat transfer mechanism and the flow boiling instabilities are explained based on the existent flow boiling regime. High speed visualizations are shown to validate the explanation. (C) 2013 Elsevier Ltd. All rights reserved.