This paper proposes a look-up-table (LUT)-based background calibration to improve the linearity of the ring voltage-controlled oscillator (VCO)-based analog-to-digital converter (ADC). The nonlinearity of the tuning gain (KVCO) of the VCO is considered as that there is an individual KVCO (KVCOI) for each ADC output level. The KVCO-locked-loop (KLL) is employed to force the KVCOIs to the same reference value, thus eliminating the nonlinearity. The effectiveness of the KLLassisted linearization scheme is not affected by the matching property of the technology and the characteristics of ADC input signal. Thanks to the KLL-assisted calibration, the total harmonic distortion is suppressed by 37.8dB with the required ADC output samples of 2e18, achieving a fast linearization process.