In laser printers, the developing station forms toner images on the photoconductor drum surface. The primary concern for formation of stable toner images is obtaining smooth developer flow. Then, observation of the developer flow at a free surface in an actual developing station has been done. For better flow optimization, developer flow details such as local flow velocity vectors and streamlines, not only at the free surface but also in the developer flow, should be clarified. A simulation method for optimizing the flow was examined.In the simulation method, a large calculation domain in the developing station is required. Therefore, viscous fluid analysis to minimize the calculation load is employed. Viscosity should be defined as fitting the developer particle flow. To do this, viscosity measurements were carried out. It was found that the developer flow should be non-Newtonian fluid, in which viscosity is proportional to the reciprocal of its shear rate. This means that shear stress in the developer flow should be constant.This property was used to simulate the developer particle flow behavior successfully, even in three-dimensional models. Flow details, on both large and small scales in the developing station, could be observed.