A micro heat sink comprising 10 microchannels integrated with a local heater and a temperature miciosensor array has been fabricated on a silicon wafer using standard micromachining techniques. A glass wafer was anodically bonded to the silicon wafer in order to cap the trapezoidal microchannels, about 14μm in depth and 120μm in average width. DI water was pressurized through the heat sink serving as the working fluid. Boiling curves of device temperature, at a few locations along the centerline, were measured as a function of the input power. In contrast to previously reported results, the boiling plateau associated with latent heat of phase change from liquid to vapor was detected. The transparent glass ceiling allowed the visualizations of flow phenomena dominated by surface effects. The classical bubble dynamics of bubble formation, growth, and detachment was observed at relatively low input power. However, this mode was completely suppressed at moderate power levels. Further increase of the input power resulted in a clear separation between the upstream vapor and the downstream liquid. The average location of the vapor/liquid interface shifted downstream with the input power, and near the critical heat flux condition the interface was located at the channel outlet. Thus, the added heat resulted in increased quality of the two-phase flow rather than increasing the mixture temperature.