Monitoring blood flow rate inside prosthetic vascular grafts enables an early detection of the graft degradation, followed by the timely intervention and prevention of the graft failure. This paper presents an inductively powered implantable blood flow sensor microsystem with bidirectional telemetry. The microsystem integrates silicon nanowire (SiNW) sensors with tunable piezoresistivity, an ultralow-power application-specific integrated circuit (ASIC), and two miniature coils that are coupled with a larger coil in an external monitoring unit to form a passive wireless link. Operating at 13.56-MHz carrier frequency, the implantable microsystem receives power and command from the external unit and backscatters digitized sensor readout through the coupling coils. The ASIC fabricated in 0.18-μm CMOS process occupies an active area of 1.5 × 1.78 mm$^2$ and consumes 21.6 μW only. The sensors based on the SiNW and diaphragm structure provide a gauge factor higher than 300 when a small negative tuning voltage (−0.5–0 V) is applied. The measured performance of the pressure sensor and ASIC has demonstrated 0.176 mmHg/√Hz sensing resolution.