This paper suggests an optimization model for the minimum cost design of rigid connections in steel structures. The cost effectiveness of optimum detail-designs based on both ASD and LRFD methods is investigated by comparing the results with those of conventional designs. In this study, three most distinct types of general rigid connections are included in the modelling, which is implemented into a computer code. The cost functions of connections are constructed by using the estimated unit costs of bolting, welding and connection-steel elements incorporating all the effects of materials, labor, and fabrication works. The behavior and design constraints are formulated based on the ASD and LRFD criteria of the AISC specifications. The flexible tolerance method is found to be very efficient as NLP algorithm for the connction optimization problems of this study. Based on numerous applications to design examples, it may be concluded that the optimization model suggested in the paper provides economical but reliable connection design with a great deal of the cost savings.