A three-dimensional (3-D) process simulator, OPUS/3D, has been developed. It has access to two-dimensional (2-D) process simulation results. Impurity profiles and structural data simulated rigidly in 2-D space are expanded to 3-D space at an arbitrary stage of the simulation processes. 3-D simulation follows until the end of the process. The access to 2-D simulation results enables OPUS/3D to handle curved boundaries as seen in field oxides. OPUS/3D has three different methods for expanding 2-D results. Errors due to these expansions are discussed. OPUS/3D was applied to 3-D simulations of MOS devices at the corner edge of the active region and in the channel region near the source/drain. Computation time was drastically reduced by replacing part of the 3-D simulations by 2-D simulations. Some 3-D effects are confirmed by OPUS/3D.>