Traditional text-based instruction may not effectively inspire the motivation for learning especially for those young students attending K-12 schools. However, the booming of mobile devices and multimedia technologies can significantly enhance the effectiveness of the learning process and strengthen student engagement. In this work, we propose a novel mobile knowledge learning system based on Augmented Reality (AR) technology to improve the learning experiences for many users. In our AR system, virtual entities are created and superimposed over real-world images or video streams. Appeared to exist in the real world defined by an image or a video, these virtual entities can directly interact with real-world objects and respond to human activities. Depending on the source of camera input, which can be a static image or a video stream, the proposed mobile AR system supports both the demonstration of physical concepts and the rendering of 3D models. We evaluate the performance of the proposed system on the efficiency and effectiveness of the rendering of virtual AR entities under various conditions. Experimental results demonstrate our system supports real-time AR rendering and provides highly interactive learning experiences for different types of users including K-12 students.