Automatic bridge detection is an important application of SAR images.Differed from the classical CFAR method,a new knowledge-based bridge detection approach is proposed.The method not only uses the backscattering intensity difference between targets and background but also applies the contextual information and spatial relationship between objects.According to bridges’ special characteristics and scattering properties in SAR images,the new knowledge-based method includes three processes:river segmentation,potential bridge areas detection and bridge discrimination.The application to AIRSAR data shows that the new method is not sensitive to rivers’ shape.Moreover,this method can detect bridges successfully when river segmentation is not very exact and is more robust than the radius projection method. Automatic bridge detection is an important application of SAR images. Differed from the classical CFAR method, a new knowledge-based bridge detection approach is proposed. The method not only uses the backscattering intensity difference between targets and background but applies the contextual information and spatial relationship between objects. According to bridges’ special characteristics and scattering properties in SAR images, the new knowledge-based method includes three processes: river segmentation, potential bridge areas detection and bridge discrimination. application to AIRSAR data shows that the new method is not sensitive to rivers’ shape.Moreover, this method can detect bridges successfully when the river segmentation is not very exact and is more robust than the radius projection method.