Negatively charged ultrathin Al2O3 film deposited by atomic layer deposition (ALD) technique has been widely applied in photovoltaic industry because of its excellent passivation effect on both p and n type silicon surfaces. In this study, we have observed an enhancement of the Al2O3 passivation on the surface of p type gallium doped Czochralski silicon under illumination, with a rapid effective lifetime increase by similar to 90%. However, such passivation effect is not stable in dark circumstance at room temperature, with a slow effective lifetime degradation to the initial value. Furthermore, we have confirmed that the enhancement process under illumination and the degradation process in dark are both thermally activated with activation energies of 0.31 +/- 0.02 eV and 0.50 +/- 0.07 eV, respectively. The capacitance-voltage (C-V) measurements based on the Al/Al2O3/p-Si capacitor demonstrate that the negative charge at the interface between ultrathin Al2O3 film and p-Si substrate increases from 7.4 x 10(11) cm(-2) to 9.1 x 10(12) cm(-2) after 1 h's illumination and then returns to the initial value in dark at room temperature after 3 h. This light induced enhancement effect is wavelength dependent and the cut-off wavelength is in the range of 780-800 nm. Based on these results, we have proposed a model to explain the light-soaking enhanced passivation of Al2O3 on crystalline silicon surface based on a charge trapping and detrapping process.