On-current (Ion) variation in metal gate FinFETs is comprehensively investigated with regard to the contributions of threshold voltage (Vt), parasitic resistance (Rpara) and trans-conductance (Gm) variations, which are successfully extracted as the independent variation sources. It is experimentally confirmed that the Gm variation of the FinFETs exhibits a linear relationship in Pelgrom plot as well as the Vt variation, and is not reduced with scaling the gate dielectric thickness unlike the Vt variation. Perspective for beyond 20nm represents that the Gm variation will be the dominant Ion variation source. A solution to reduce the Gm variation for the FinFET is also proposed.