Abstract: High precision time-to-digital converter (TDC) in positron emission tomography (PET) system was studied with the delay chains in field-programmable-gate-array (FPGA). Time measurement includes two parts, coarse time measurement implemented by a clock-driven counter, fine time measurement by delay chains in FPGA. Differential nonlinearity (DNL) and integral nonlinearity (INL) were tested, and also, total time resolution of the double-detector PET experimental platform was measured by time coincidence. The results demonstrate that the time resolution of TDC reaches 79.3 ps, and the DNL is between -0.2 LSB and 0.2 LSB, the INL is between-0.2 LSB and 0.3 LSB, the total time resolution of the double-detector PET experimental platform reaches 2.1 ns. Performance of the TDC, implemented in FPGA, can meet the demand of time measurement in PET system.