An experimental study of the heater size effect on micro boiling is reported in detail. Using a 1.66-ms-wide heating pulse, boiling in subcooled water was investigated on a series of micron/submicron thin film Pt heaters with various feature sizes ranging from 0.5 mu m to 70 mu m. It was found that there existed a critical heater size (10 mu m): single spherical bubble generation with heater's feature size less than 10 gm; oblate vapor blanket on the heater surface with the size larger than 10 mu m. The bubble dynamics was studied by the visualization of the bubble nucleation process with a high-speed CCD. The onset bubble nucleation temperature was measured by using each Pt heater as a resistive temperature sensor. The formation of the oblate vapor blanket was attributed to the condensation effect of the vapor outside the superheated zone. The analysis was further validated by generating spherical bubble on heater with size larger than 10 mu m with a longer heating pulse. (c) 2006 Elsevier Ltd. All rights reserved.