The random work-function (WK) induced threshold voltage fluctuation (sigmaVth) in 16 nm Titanium Nitride (TiN) metal-gate fin-type field effect transistors (FinFETs) is explored and modeled by using an experimentally validated Monte Carlo simulation approach. The influences of metal-grain size and device geometry aspect ratio on the random WK-induced sigmaVth are considered in the proposed equation analytically. The formula accounts for the inside of fluctuation and could be used for the assessment of effectiveness of suppression techniques.