Experiments were conducted to investigate flow boiling heat transfer of R134a in a multi microchannel heat sink. The heat sink consisted of 25 rectangular microchannels with nominal dimensions of 300 mu m wide, 700 mu m deep (D-h = 420 mu m) and 200 mu m separating wall thickness. The heat sink was made of oxygen free copper by CNC machining and was 20 mm long and 15 mm wide. The experiments were conducted at 6.5 bar system pressure and covered a footprint area-based heat flux range 11.46-403.1 kW/m(2) and mass flux range 50-300 kg/m(2) s. A high speed camera was used to capture the flow patterns simultaneously with heat transfer measurements. Three flow patterns were observed namely bubbly, slug and wavy-annular flow when the heat flux increased gradually. The heat transfer coefficient increased with heat flux and there was no mass flux effect. Assessing existing correlations indicated that the correlations of Mahmoud and Karayiannis (2013) and Cooper (1984) predict the data very well with a mean absolute error less than 20% compared to the other correlations. (C) 2017 The Authors. Published by Elsevier Ltd.