We have studied dynamics of magnetic brushes in two-component development system of electrophotography by a numerical simulation with an improved Distinct Element Method (DEM) and experimental observation with a high-speed microscope camera. Two-dimensional magnetic field was calculated by superposing measured discrete magnetic flux densities on a development sleeve. It has been calculated and observed that magnetic brushes grow up in the development area inclining in parallel in the direction of the magnetic field and they are crushed by a photoreceptor drum. It has been evaluated that although the total pressure applied to the photoreceptor is almost irrelevant to the diameter of carrier particles, differential force applied by individual chain is dense and small with small particles but rough and large with large particles. This suggests that small carrier particles are preferable to prevent disturbance of developed toner particles on the photoreceptor.