This work proposes a wireless operating system for continuous monitoring of glucose level. The system consists of an implantable device and an external readout transmitter. The implantable device measures a glucose level and wirelessly communicates with the external reader, employing the backscattered modulation for low-power operation. The implantable device converts the light intensity scattered from a fluorescent hydrogel sensor reacting on glucose level to the frequency-modulated signal, and sends this back to the external reader while it is being powered wirelessly. The entire implantable device including the 1000×600-μm² die fabricated in TSMC 180-nm CMOS process is bonded on PCB and the fluorescence hydrogel sensor fits in 2×2.5 mm2, and consumes 146.5 μW of power in average during the operation.