—This paper presents an oscillator-based true random number generator (TRNG) that automatically adjusts the duty cycle of a fast oscillator to 50 %, and generates unbiased random numbers tolerating process variation and dynamic temperature fluctuation. Measurement results with 65nm test chips show that the proposed TRNG adjusted the probability of ‘1’ to within 50 ± 0.07 % in five chips in the temperature range of 0 ◦ C to 75 ◦ C. Consequently, the proposed TRNG passed the NIST and DIEHARD tests at 7.5 Mbps with 6,670 μm2 area.