We report a new silicon MEMS accelerometer based on differential Frequency Modulation (FM) with experimentally demonstrated self-calibration against dynamic temperature environment and μg-level Allan deviation of bias. The sensor architecture is based on resonant frequency tracking in a vacuum packaged SOI tuning fork oscillator with a high Q-factor. The oscillator is instrumented with a DC voltage biased parallel plate capacitor, which couples the proof mass displacement to the effective stiffness by means of the negative electrostatic spring effect. External acceleration is detected as an FM signal. To address drift over temperature, the MEMS sensor die incorporates two identical tuning forks with opposing axes of sensitivity. Demodulation of the differential FM output from the two simultaneously operated oscillators eliminates common mode errors and provides a continuously self-calibrated FM output. An x-axis SOI prototype with a tunable scale factor was built and characterized over dynamic temperature environment, experimentally demonstrating continuous self-calibration.