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Abstract There is a need to accurately measure the flow rates of well fluids, in both drilling and production operations. While pairs of absolute pressure sensors could be used, a true differential pressure (DP) measurement is required for accurate downhole flow measurement, but has not previously been available. The goal of this work was to design, fabricate and test an HPHT (high-pressure, high temperature) DP sensor at borehole temperatures up to 500 Deg F and pressures up to 10,000 psi. An HPHT absolute P and a DP sensor were developed for subsea flowmeter use in the RPSEA – the Research Partnership to Secure Energy for America – project 07121-1301. However, the diameter of that P-DP sensor cell is too large for downhole applications which require a less than one-inch diameter cell. The 1301 design was modified using solids-modeling software and prototypes were fabricated. Because of the high common-mode pressure, electron-beam welding was required to join the DP cell components together and to the pressure flanges. The completed units were calibrated at ambient conditions and then sent to a certified metrology facility for testing at HPHT conditions. Calibration of the DP cell showed perfect linearity and good overall accuracy. The maximum error at each of the extremes (500 Deg F and at 10,000 psi) was measured as 0.02% of upper-range limit, even without data processing to make this a "smart" DP transmitter. The cell's overpressure protection functioned to prevent damage to the MEMS DP chip at the extreme conditions. Care must be taken in the electron-beam welding step of the fabrication to prevent the sensor chip's temperature from exceeding its maximum rating. The development demonstrated the ability of the design to meet the requirements for a true HPHT DP sensor for downhole applications. The measurement capability of a small diameter, true HPHT DP sensor will be of benefit in normal production flow measurement, but it could also be valuable in early detection of a well inflow event during drilling. Observing loss of well control as it is happening could mean the difference between a safe operation and one that is not. Additionally, the DP sensors could also be located at a fixed downhole location, at the sea floor, or at intermediate points (e.g. zones) in the well.