Abstract: A chevron bracing structural system is widely used in multi-story and high rise steel constructions. While under severe seismic actions, a significant unbalance force in the vertical direction may form at the intersect point of the braces due to the buckling of the compressive brace. This unbalance force, acting at the mid-span of a beam, may lead to the failure of the beam and, consequently, the dramatic decrease of the lateral force resistance of the structural system. Many of the current design codes worldwide require the beam of such a bracing system should have the capacity of resisting such an unbalance force. However this requirement may not be satisfied in some extent in practice, indicating that the behaviour of the chevron bracing systems with a weak beam are still of important. An investigation is carried out in this paper on a chevron bracing system with a weak beam, where the out-of-plane slenderness of the bracing members are varied from 20 to 150 and four types of beams with the load carrying capacities respectively being 0.3, 0.5, 0.8 and 1.0 time that of the beam with adequate loading resistance under the vertical unbalance force are considered. The overall behaviour, the respective behaviours of the braces in compression and tension, the overstrength coefficient and the ultimate load of a chevron bracing system with a weak beam under lateral loads are concerned. The study is believed to be helpful for understanding the behaviour of a chevron bracing system with a weak beam under severe seismic loads