Two-step algorithm is proposed for unsupervised detection of linear structure from SAR image, in particular, the road network detection. The first step is to detect line element with local detector, in this paper an improved ratio of average (RoA) detector which takes into account the inner statistic property of SAR image is proposed. It can guarantee the detection accuracy. In the second step, we introduce the prior knowledge about the road structure to group the line element into macroscopical structure. The method proposed is identified in ERS-1 image.