A simulation method using 2-D and 3-D models is proposed for analysis of primary transfer processes. In the method, the motion of toner particles between an organic photoconductor (OPC) and an intermediate transfer belt (ITB) is calculated with a 3-D parallel plate model, while the 2-D electric potential distribution of the ITB is used as a boundary condition. The electric potential distribution of the ITB is obtained by 2-D electric field calculation, where a vicinity of a transfer nip is modeled with a boundary-fitted coordinate system, and the Poisson equation is solved in consideration of the transfer bias, transfer member configurations, the Paschen discharge, electrical conduction and advection of electric charge. In the 3-D calculation, a space between the OPC and the ITB in a limited part of an image area is modeled in a 3-D parallel plate model, and the motion of toner particles, which obeys Newton's second law of motion, and the Poisson equation are solved in consideration of the electric potential of the belt, charge and adhesion of toner, the latent image on the OPC, and the Paschen discharge. The 3-D calculation is carried out with changing the transfer gap, while the electric potential of the ITB obtained by the 2-D calculation is applied to the lower surface of the ITB layer. The simulation method enables analysis of transfer processes with a low computational load, and simulated results of transfer efficiency and toner scattering show good agreement with experiments.