In this paper, we propose a power/area-efficient capacitive readout circuit for the Micro-Electro-Mechanical Systems (MEMS) sensor of 3-axis accelerometer. The proposed architecture is different from other traditional structures by exploiting true capacitive-to-digital converter (CDC) without Analog-to-Digital Converter (ADC). The proposed CDC can differentiate the bidirectional 125KS/s 80-level accelerations between ±8g and support the 4-level adjustable resolutions of 0.1g/0.2g/0.4g/0.8g for each axis. The proposed readout circuit with 0.0354mm2 area is fabricated in UMC 0.18um CMOS-MEMS process. Experimental results show power consumption is 50uW with 1.8V supply voltage for 1-axis (FOM=3.84pJ), 82uW for 3-axis (FOM=2.1pJ) under 125KHz of sampling frequency and 0.1g acceleration sensitivity for 0.2fF MEMS capacitance change.