An out-of-plane capacitive accelerometer with low cross axis sensitivity at high accelerations based on a CMOS MEMS process is developed. A truss frame with a high ratio of in-plane stiffness to out-of-plane stiffness is designed to suspend the proof mass of the accelerometer. The configuration of the proof mass with four auxiliary masses surrounding a main mass boosts the sensitivity of the accelerometer in the out-of-plane direction. A sensing circuit with period modulated output signals is realized by applying a relaxation oscillator to the sensing capacitors. The relaxation oscillator converts the capacitance changes to frequency variations. Analytical models are developed to assist the design of the accelerometer. The nonlinearity and sensitivity of the accelerometer are 1.1% and 754 Hz/g, respectively, for the acceleration ranging from 0 to 9 g. Measured cross-axis sensitivity is 4% at an acceleration of 9 g. The power consumption is 1.7 mW under a 1.8 V supply.