We report on the electric-field-induced reversible metal-insulator transition (MIT) of the insulating LaAlO3 thin films observed in metal/LaAlO3/Nb-SrTiO3 heterostructures. The switching voltage depends strongly on the thickness of the LaAlO3 thin film which indicates that a minimum thickness is required for the MIT. A constant opposing voltage is required to deplete the charges from the defect states. Our experimental results exclude the possibility of diffusion of the metal electrodes or oxygen vacancies into the LaAlO3 layer. Instead, the phenomenon is attributed to the formation of a quasi-conduction band (QCB) in the defect states of LaAlO3 that forms a continuum state with the conduction band of the Nb-SrTiO3. Once this continuum (metallic) state is formed, the state remains stable even when the voltage bias is turned off. The thickness dependent reverse switch-on voltage and the constant forward switch-off voltage are consistent with our model. The viewpoint proposed here can provide an alternative mechanism for resistive switching in complex oxides. © 2011 American Physical Society.