The behaviour factor ( q -factor) is a factor introduced in Eurocode-8 (EC8) to reduce the seismic elastic forces of structures obtained from an elastic analysis accounting for their ductility and overstrength. Seismic codes, especially the EC8, gives a constant value for q -factor, since change in structural characteristics of building change in behaviour of braced steel structures and that affects on q -factor. In this paper, the q -factor is evaluated for X-braced steel structures using pushover analysis. The effects of brace slenderness ratio and stories number are investigated. The results of this study indicate that the most important parameter that affects the q -factor is the brace slenderness ratio. Furthermore, the EC8 recommends a higher than actual value of q -factor, which is potentially unsafe.