Abstract: In the present paper, the author summarizes what is thought about the main characteristics of shear folds by some researchers. On the basis of available data, the main characteristics of shear folds are dealt with briefly from six aspects, and their genesis is further discussed. It is believed that the deformation of the shear fold belongs to flattening deformation, and its genesis may be called flattening folding. Where layered rocks （which are often plastic,coherent and thin-layered） are subjected to strong compression along bedding planes.the rocks are flattened continuously. Instead of bending, they shorten and thicken a lot. At the same time, some minerals, primary and newly formed, are oriented in the direction perpendicular to the compressional stress. Along latent planes of weakness, elastic strain transforms slowly and gradually into permament strain. Consequently, a series of parallel and close-spaced ductile ruptures（flow cleavages） are formed, but they are usually inclined to the bedding planes. After the formation of the cleavages（the continuity of the stratification is destroyed）, further compression shows itself as flattening perpendicular to the cleavages. Owing to the heterogeniety of flattening （the minerals are often forced out n the zone of the strongest flattening）, the strata may slip differentially （?）nd regularly （Fig. 9-b）, and then shear folds are formed. Because of the difference in the boundary condition as the shear folos are formed, the concrete formation process may show itself in two different types which are also covered in this paper.