We have investigated dynamics of a magnetic brush of carrier particles in the two-component development system of electrophotography by the numerical simulation with an improved Distinct Element Method and the 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 chains of the brush grow up in the development area inclining in parallel in the direction of the magnetic field and they are crushed by a photoreceptor drum. The brush is depressed to backward before the center of the gap and springs just before the brush become free. The spring-back of the brush at the backward of the development area seems to cause a stripe defect of the developed toner images on the photoreceptor if the force applied by the brush to the photoreceptor is excessive. The differential force applied by individual chain is dense and small with small particles. On the other hand, it is rough and large with large particles. These suggest that small carrier particles are preferable to prevent disturbance of developed toner particles on the photoreceptor. The effects of the development gap and the thickness of supplied carrier particles have also evaluated.