A gated ring oscillator (GRO) based time-to-digital converter (TDC) is presented. To enhance the resolution of the TDC, a multi-path structure for the GRO is used to achieve a higher oscillation frequency and an input stage is also presented to equivalently amplify the input time difference with a gain of 2. The GRO based TDC circuit is fabricated in TSMC 65 nm CMOS technology and the core area is about 0.02 mm2. According to the measurement results, the effective resolution of this circuit is better than 4.22 ps under a 50 MHz clock frequency. With a 1 ns input range, the maximum clock frequency of this circuit is larger than 200 MHz. Under a 1 V power supply, with a 200-800 ps input time difference, the measured power consumption is 1.24 to 1.72 mW at 50 MHz clock frequency and 1.73 to 2.20 mW at 200 MHz clock frequency.