Abstract: Compresso-shear faults have been repeatedly found among numerous nor- thwest-trending faults in eastern South Jiangsu and the Shanghai area. Acco- rding to the strike, the northwest-trending faults may be divided into two sets, the northwest set and the north-northwest set. Through an analysis, the mechanical nature of structure planes could be determined relatively accu- rately. The development of the northwest set of compresso-shear faults pro- gressed through three stages: （1） compressive or compressoshear activity, which originated in early Yanshanian time and was still operative in late Yanshanian time; （2） tensile activity; and （3） shear activity. The tensile and shear activity of the northwest-trending compresso-shear faults and the north-northwest-trending compresso-shear faults occurred in late Yanshanian time. The tensile and shear faults of the northwest set, and the tensile and shear activity of the northwest-trending compresso-shear faults are genetica- lly related to the Cathaysian and Neocathaysian tectonic systems, belonging to lowergeneration elements of the two tectonic systems; while the compresso -shear faults may possibly belong to another independant tectonic system of shear origin which is very important. The study of the northwest-trending faults is of great significance in the study of regional geology, metallic mineral deposits, coal deposits, clay deposits and present-day seismic activity.