The interfacial dipole and bulk trap in HKMG stack have been found to be significant to the work function variation (σVWF), in addition to the metal grains. In order to differentiate their effects on σVWF, a new variation plot is proposed and the dipole and trap effects can be distinguished. Here, we propose a simple experimental method to separate the effects of MG/HK and HK/IL interfacial dipoles. In pMOSFET, HK/IL dipoles dominate HK induced variation; MG/HK dipoles are dominant in nMOSFET. However, in terms of the reliability test, after PBTI stress, HK bulk traps play a major role in the variation of nMOSFET, while after NBTI, HK/IL dipoles are strengthened by hydrogen bonds and still dominant in work function variation of pMOSFET. Design guideline is provided to deal with the passivation of high-k traps by nitrogen concentration and the improvement of variability in HKMG CMOS devices.