With the progress of LSI technology, the electronic device size is presently scaling down to the nano-meter region. In such an ultrasmall device, it is indispensable to take quantum mechanical effects into account in device modeling. In this paper, we first review the approaches to the quantum mechanical modeling of carrier transport in ultrasmall semiconductor devices. Then, we propose a novel quantum device model based upon a direct solution of the Boltzmann equation for multi-dimensional practical use. In this model, the quantum effects are represented in terms of quantum mechanically corrected potential in the classical Boltzmann equation.