Direct tunneling of thin SiO2 films on n-type Si(100) is calculated by the Wentzel, Kramers, Brillouin (WKB) method considering the ground energy level which corresponds to the four-folded valleys and the effective SiO2 thickness by the quantum effect, The calculated results reproduce the measured data fairly well except for the low applied voltages. Furthermore, the reproducibility of the I-V characteristics in the low voltages increased by considering the modulation of the effective electron mass in the SiO2. (C) 2002 Published by Elsevier Science Ltd.