A surface-micromachined, high-pressure sensor with polysilicon piezoresistors for oil well applications has been successfully developed. This work represents the first report of a nitride-diaphragm (3μm thick) oil-well pressure sensor with a diaphragm diameter of 44μm. The sensors have been calibrated up to 4000 psi. A sensitivity of 7.7 μV/V/psi is obtained. With 5 V excitation voltage, the full scale output (FSO, 4000 psi) is 154 mV. The nonlinearity is less than ± 0.2% of FSO. The hysteresis is less than 0.04% of FSO. A temperature coefficient of 25 μV/V/°C (equivalent to 0.08% of FSO/°C) is observed under 1000 psi pressure bias.