This paper presents the results of an experimental investigation of flow boiling heat transfer in multi microchannel heat sink. The test section, 20 mm long and 15 mm wide, consisted of 26 rectangular microchannels (350 μm wide, 630 μm deep -i.e. Dh = 450 μm - and 200 μm wall thickness) produced by CNC machining from an oxygen free copper block. Using R134a as a test fluid, the experiments were conducted at a system pressure of 7 bar, heat flux range 7.6–625.8 kW/m2 and mass flux range 50–300 kg/m2s. A high speed camera was used to capture the flow patterns. Four flow patterns were observed namely bubbly, slug flow/confined bubble, churn flow and annular flow as the heat flux gradually increased. Results show that the maximum heat transfer coefficient obtained is 35.52 kW/m2K.The results also show that the heat transfer coefficient increased with heat flux and there was no mass flux effect. Relevant literature correlations predicting the heat transfer coefficient were evaluated against the current data