Experiments were conducted to measure the onset of flow instability (OFI) (static flow instability) and dynamic unsteady flow in a compact heat sink consisting of 26 rectangular microchannels with 300 mum width and 800 mum depth. The planform area is 5.0 x 1.53 cm(2). Tests were performed with water and methanol as working fluids with mass fluxes of 20-1200 kg/m(2) s, Inlet temperatures of 30, 50 and 70 degreesC and effective heating powers of 100-450 W. It was found that the onset of flow instability occurs at the outlet temperature of 93-96 degreesC, which was several degrees lower than the saturated temperature of 100 degreesC corresponding to the exit pressure. Fine bubbles were detected close to the channel exit, and they were moving out of the channel to form a unit circular bubble in the outlet plenum. Once the mass flux is lower than that of OFI, three types of oscillations were identified: large amplitude/long period oscillation superimposed with small amplitude/short period oscillation and small amplitude/short period oscillation. Thermal oscillations were always accompanying the above two oscillations. Observations through the microscope and physical explanations were given corresponding to the above oscillation flows. Similar tests were also performed with methanol as working fluid. (C) 2004 Elsevier Ltd. All rights reserved.