Heat transfer from a micro heater is investigated experimentally and numerically. The micro heaters made of Indium Tin Oxide (ITO) are set on a glass substrate immersed in FX3250 coolant. Two different sizes (50μm, 100μm) of heaters are used in the experiments to analyze the size effect. The effect of the surface deposited layer by Al2O3 is also investigated with changing the thickness of the layer. All the experiments are performed in both subcooled and saturated boiling. The boiling curves of different conditions are obtained and compared. The results show that there exists an optimum thickness of deposited layer for heat transfer enhancement. They also show that the present heat transfer process is quite different from that of usual boiling. The phenomena are dominated by heat conduction in the substrate and large portion of the generated heat is transferred to the liquid and then vapor bubble through the substrate around a heater. Further, a photograph of a liquid microlayer under a vapor bubble is taken and the relation between an unsteady bubble behavior and local temperature fluctuations is analyzed.