Abstract: Very thin and flat silicon sensitive diaphragm is a most important part for fabricating micro-machined capacitive differential pressure sensors with low measure range. In this paper, a simple fabrication method of such thin diaphragm was described in details. During the fabrication of such diaphragm, the key technologies of heavy-doped B diffusion, KOH etch-stop and LPCVD were used. Results show the 4μm thickness compound diaphragm made of Si（P++） and Si₃N₄ is very flat which meets the requirement of the sensor, and its deflection is about several hundreds nanometers.