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This paper reports the development of piezoresistive accelerometers based on an asymmetrically gapped cantilever which is composed of a bottom mechanical layer and a top piezoresistive layer separated by a gap. The asymmetrically gapped cantilever helps to increase the sensitivity and enables the majority of mechanical energy to be effectively used to strain the piezoresistive layer. An analytic model of the asymmetrically gapped cantilever was developed and verified using finite-element simulation. Design optimization was discussed based on the analytical model. A figure of merit was defined as the product of the signal-to-noise ratio and resonant frequency. It was demonstrated that the energy efficiency is a critical criterion of design optimization. The prototypes of the piezoresistive accelerometer were successfully fabricated using deep reactive-ion etching from both the front and back sides of silicon-on-insulator wafers. The fabricated devices were preliminarily characterized. A sensitivity of 0.36 mV/V/g and a fundamental resonant frequency of 4060 Hz were obtained. The noise of the fabricated device was also measured and analyzed.