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. Accordingly, observation of the developer flow at a free surface in an actual developing station has been carried out. 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 is employed to minimize the calculation load. Viscosity should be defined in terms of the developer particle flow. To do this, viscosity measurements were carried out. It was found that the developer flow should be non-Newtonian, whereby viscosity is proportional to the reciprocal of its shear rate. This means that shear stress in the developer flow should optimally be constant. This property was used to simulate the developer particle flow behavior successfully, even in three-dimensional models. Flow details in the developing station, on both large and small scales, could be observed.