A lot of studies on the optimal structural design of building frames or trusses have used structural weight as a cost function.However, it is often that structural weight is not effective as a cost function, because the fabrication cost of the steel members depends on the complexity of the connections rather than the structural weight.A cost function considering the nodal cost for truss optimization (Ohsaki 1995) and some fabrication time functions (Jarmai and Farkas 1999, Pavlovcic et al.2004) have been proposed previously.Recently, a relatively simple fabrication time function for the rigid frames of steel buildings has been proposed (Sasaki et al.2007).In this paper, minimum cost frames considering the simple fabrication time function proposed by Sasaki et al.(2007) are compared with minimum weight frames under constraints based on Japanese seismic code.Here, Genetic algorithm is used as an optimization algorithm.Seismic responses of both frames are compared by means of time history analyses.In numerical examples, inner beams and outer beams of the minimum weight frame are not same in depth, while inner beams and outer beams of the minimum cost frame are same in depth (Fig.1).It is observed from time history analysis that seismic responses of the minimum cost frame are slightly larger than those of the minimum weight frame (Fig.2).There are differences in R, the ratio of member end moment to plastic moment under the seismic design load, between outer beam and inner beam in the minimum cost frame (Fig.3).The difference causes damage concentration on the middle story inner beams of the minimum cost frame.