Abstract: Poor quality in some soybean seed during storage can be due to their physiological characteristic or mechanical causes. The soybean seeds are susceptible to mechanical injury mediated damage during post harvest handling. These injuries lead to deterioration during storage. The purpose of this investigation was to evaluate the effects of different threshing methods i.e., hand threshing, beating by stick threshing and mechanical threshing on some soybean cultivars i.e., Giza 21 (G-21), Giza 35 (G-35) and Giza 111 (G-111) yield, seed quality and its longevity during storage. A field experiment was carried out at the Experimental Farm of Agronomy Department, Faculty of Agriculture, Mansoura University, Egypt during 2010 and 2011, followed by a laboratory experiment under the laboratory conditions of Seed Technology Research Unit in Mansoura, Dakahlia Governorate, Seed Technology Research Department, Field Crops Research Institute, Agricultural Research Center. Seed quality tests were performed on each treatment, directly after threshing (0 month), three or six months storage. The results showed significant differences among soybean cultivars in all studied traits. G-21 surpassed G-35 and G-111 cultivars on seed yield and related attributes, oil and protein %, germination, germination after accelerated aging %, seed and seedling vigor. Threshing methods insignificantly affected oil and protein percentages, but exhibited significant effect on seed yield and seed quality. Threshing by hand or by stick methods were reduced seed quality less than mechanical method. Seed viability of soybean cultivars decreased gradually with increasing storage period up to six months. G-111 showed lower resistance to storage conditions, which might be classed as poor "storer" under the experimental conditions. Higher viability of G-21 seed (as indicated by germination score) than others (G-35 and G-111) might be classified as a good "storer". Assessment of some soybean cultivars seed quality during storage by monitoring germination and germination after aging percentages, in addition to seed and seedling vigor measurements may be reliable indicators for damages occurred after mechanical threshing method.