Among various traditional art forms, brush stroke drawing is one of the widely used styles in modern computer graphic tools such as GIMP, Photoshop and Painter. In this paper, an AI-aided art authoring (A4) system of non-photorealistic rendering is developed that allows users to automatically generate brush stroke paintings in a specific artist's style. Within the reinforcement learning framework of brush stroke generation, the first contribution in this paper is the application of regularized policy gradient method, which is more suitable for the stroke generation task. The other contribution is to learn artists' drawing styles from video-captured stroke data by inverse reinforcement learning. Experiments demonstrate that the presented system can successfully learn artists' styles and render pictures with consistent and smooth brush strokes.