A low-power and high accuracy comparator based on voltage controlled ring-oscillator (VCO) is presented. By using the dead zone of phase detector effectively, the VCO comparator automatically changes its noise level depending on the input voltage level (Δvin). When Δvin is large, the comparator operates as a low-power delay-line based comparator. On the other hand, when Δvin is small, the VCO is enabled and eye is opened during the oscillation. This suppress input referred noise and enables accurate conversion. The number of oscillation cycle for one comparison is inversely proportional to Δvin and adaptive noise reduction is realized. The VCO comparator does not require any sort of tuning. A 13b SAR ADC with proposed VCO based comparator was fabricated in 65-nm CMOS. By off-chip LMS calibration, the ADC achieves SNDR 66 dB at 1 MS/s with FoM of 29fJ/conv.-step.