The rendering of lines for 3D models is an important research topic in NPR(Non-Photorealistic Rendering). In line drawing, the use of loose style can help artists express their emotion and depict objects vividly. In this paper, we summarized the aesthetic factors of loose line style in two aspects: line thickness and line omission, and proposed a mathematical description for it. We also provided a real-time, temporal coherent rendering method for loose style lines. The subjective tests showed that our rendering results are sufficiently aesthetic pleasing.