We have been studying on subthreshold characteristics of SOI MOSFETs in terms of substrate bias dependence, using a 1-D Poisson equation on an SOI multi-layer structure for estimating structural parameters of real devices[1]. Here, we consider quantum mechanical effects in the electron inversion layer of thin SOI MOSFETs, implementing a self-consistent solver of Poisson and Schrödinger equations in a 1-D subthreshold simulator. From results of simulations, we have concluded that quantum mechanical effects need to be considered in analizing thin SOI devices.