In this study, the work function fluctuation (WKF) induced device variability in 16-nm-gate bulk and SOI FinFETs is for the first time explored by using an experimentally calibrated three-dimensional (3D) device simulation. Random nanosized grains of TiN gate are statistically positioned in the gate region of device to examine the associated electrostatic and carriers' transport properties, concurrently capturing random grain's size, number and position fluctuations. Both bulk and SOI FinFETs with TiN/HfO2 gate stack are simulated, based upon experimentally available data. The approach of localized WKF simulation method is thus intensively performed to explore the device's variability including comparison between bulk and SOI FinFETs. The results of this study enable us to get an even reasonably accurate account of the random grain's number, position and size effects.