In this paper, a theoretical analysis of current-controlled (CC-) MOS current mode logic (MCML) is reported. Furthermore, the circuit performance of the CC-MCML with the auto-detection of threshold voltage (V_th) fluctuation is evaluated. The proposed CC-MCML with the auto-detection of V_th fluctuation automatically suppresses the degradation of circuit performance induced by the V_th fluctuations of the transistors automatically, by detecting these fluctuations. When a V_th fluctuation of ±0.1V occurs on the circuit, the cutoff frequency of the circuit is increased from 0Hz to 3.5GHz by using the proposed CC-MCML with the auto-detection of V_th fluctuation.