A surface-micromachined silicon accelerometer with a novel concept, which has a stiffness tuning capability to improve the sensor resolution, is developed. Imposing an electrostatic force to the electrodes reduces the stiffness of the sensor structure. By adopting the stiffness tuning, the initially stiff structure guarantees the stability of fabrication, and the reduced stiffness, only along the sensing direction, produces the improved resolution. One of the major improvements in the developed accelerometer is the branched comb-finger type electrode which senses the relative position between the mass and the electrode. Maintaining the same capacitance variation, such electrodes allow a larger initial gap between the mass and the electrode, so that the clash problem can be easily eliminated. The accelerometer was successfully fabricated with the active size of 650 x 530 mu m(2), the 7-mu m thick polysilicon structure, and a proof mass of about 1 mu g. Experimental results show that the equivalent noise level of the accelerometer is improved by 30 dB through the stiffness tuning. The accelerometer has the bandwidth of 350 Hz, linearity of 0.3% FS, and sensing range of 50 g. (C) 1999 Elsevier Science S.A. All rights reserved.