A 6.29 nW temperature-to-digital converter for infrared (IR) radiation-based thermopile temperature sensors was designed and fabricated on the chip area of 0.068 mm^2 using the 90 nm CMOS technology. The sub-mV voltage produced by the sensor nearly proportional to the temperature difference between the object and the environment is converted to sub-nA current using the gate-leakage characteristics of PMOS transistors. The ratio of the sub-nA current and the reference current proportional to the sub-mV voltage difference is converted to the frequency ratio of the two current-to-frequency oscillators. Then, the digital output proportional to the tiny voltage difference can be obtained by the counters. After the two-point calibrated digital outputs are mapped to the object temperatures, the average resolution is 0.11ºC with the conversion time of 348 ms, which results in the figure of merit (FoM) of 0.026 nJ׺C^2. The inaccuracy is +0.18/-0.98ºC for the object temperatures from -10ºC to 100ºC at the ambient temperature of 25ºC.