High resolution side-looking sonar data is of immense use in understanding the geology and the tectonics of seafloor. In order to study the detailed morphology and the crustal movement inferred from the topographic fabrics, it is necessary to analyze bathymetry as well as backscatter data. We introduce a new method ﾔseafloor stereographﾕ consists of two side-looking sonar images from slightly different viewpoints. It helps intuitive and rapid understanding of the seafloor and gives us new information and interpretations, which have not been obtained from the usual analyses. Two examples of "seafloor stereograph" are presented, which lead us new interpretation of the studied are. "Seafloor stereograph" has proved to be of great help in deciphering folds, faults and sediment structure etc. It will become powerful tool for morphological and geological analysis and for the study of seafloor tectonics.