This work, for the first time, examines the work function fluctuation (WKF) and interface trap fluctuation (ITF) using experimentally calibrated 3D device simulation on high-κ / metal gate technology. The random WKs result in 36.7 mV threshold voltage fluctuation (σVth) for 16 nm N-MOSFETs with TiN gate, which is rather different from the result of averaged WKF (AWKF) method  due to localized random WK effect. The ITF affects the subthreshold region (the normalized σID > 48%) and is suppressed for devices under strong inversion. Estimation of statistical covariance confirms the dependence of IT on the metal gate's WK; thus, the impacts of WKF and ITF on device and circuit variability should be considered together properly. Such variability induced static noise margin fluctuation of SRAM exceeds the influence of random dopants and cannot be ignored.