The scan line method is one of the most commonly used methods in rock joints survey. To ensure the quantity of collected joint samples enough, and at the same time, to save surveying workload, the optimum scan line length should be determined first of all. For the moment, the optimum scan line length is usually determined by? statistical properties of joint space, while, the authors research result indicates that the optimum scan line length does not only rely on statistical properties of joint space, but also on statistical properties of joint trace length and other joint geometry parameters. Then, a? modified method to determine the reasonably optimum scan line length is put forward, and two cases are also given in this article.