The origin of negative flat-band shift using La2O3 incorporation in HfO2 dielectrics has been extensively examined. From careful extraction of effective work function of gate electrode and fixed charges at each interface, it has been revealed that La2O3 at high-k/Si substrate or high-k/SiO2 interface has either large amount of positive fixed charges or an additional dipole of 0.36 V compared to that of HfO2/Si or HfO2/SiO2. Stacked MOSCAPs were fabricated and the C-V characteristics show that flat-band voltage shift is mainly determined by high-k film which is in contact to Si or SiO2. Using HfLaO with different La concentration, the amount of shift in flat-band voltage could be well controlled, which might be due to the diffusion or pile-up of La atoms to the interface over 420°C. This study provides further insights in controlling the threshold voltage Of HfO2 based oxides.