Rotating drum is the main part of industrial rotary kilns. The repose angle is one of the most important parameters describing the solids motion inside the drum. A real-time monitoring system is developed in this paper to detect the repose angle based on image processing techniques. The video of the solids motion is taken by a camera and transferred to a computer via an image-sampling box. With a VC++ program, the image of the solids is captured and processed through segmentation, morphology processing and edge detection. The repose angle is determined by linear fitting of the surface profile of the solids bed. With the image processing system, the dynamic change of repose angle with time can be measured and displayed automatically. The effectiveness of the measuring method is proved on a testing drum of 250mm partially filled with rice particles. Results of the present work can be expanded to the automatic monitoring of solids motion in industrial rotary kilns.