Device simulation of HEMT (High Electron Mobility Transistor) has been carried out by ensemble Monte Carlo simulation, where two-dimensional motion of electrons confined in GaAs region at GaAs/AIGaAs heterointerface is taken into account. In order to develop a realistic device simulator of HEMT, we divide the channel region in meshes, and Schrödinger and Poisson equations are solved self-consistently to obtain electronic states of the twodimensional electrons in a mesh. In addition the real space transfer from GaAs layer into AIGaAs barrier layer is taken into account.