Abstract: Using MEMS technology, a Pt microheater (60 μm×100 μm) fabricated on a glass wafer was placed in a siliconbased microchannel of trapezoidal cross section. With the aid of a highspeed CCD, flow boiling phenomena on the surface of the microheater in the microchannel under pulse heating were observed and recorded. At a given mass flux, nucleate boiling and film boiling began to appear on the microheater with increasing heat flux. A flow boiling map, showing the effects of heat and mass flux on the nucleate and film boiling regimes on the microheater at a pulse heating width of 2 ms, was presented. It is found that the nucleate boiling was changed to film boiling as the heat flux supplied by the microheater is increased. Furthermore, increasing mass flux increases the heat flux required for the incipience of nucleate boiling and film boiling on the microheater in the microchannel.