We have employed electric-field-induced second-harmonic (EFISH) generation to determine the flat-band voltage (V(FB)) of Cr/ALD-Al(2)O(3)/MBE-HfO(2)/n-Si (001) MOS structure. Due to the phase sensitivity of EFISH signal to the electric field in the space charge region, the VFB of -1.20 +/- 0.07 V was determined by analyzing the relative phase change in the EFISH signal as a function of the applied gate voltage. The obtained value is in good agreement with that estimated by the capacitance-voltage measurement. This study demonstrated an all-optical technique to directly determine the flat-band voltage for the high kappa oxide/Si heterointerfaces. (C) 2011 American Institute of Physics.