Abstract: By using rotational displacement equation of beams with varied cross-section and analyzing the effect of the column proximal end, the paper derives the equations for determining the effective length factor of columns in non-sway and sway steel frames with RBS (reduced beam section) connections. To do that, modified linear stiffness ratio is introduced, which can be written as a formula similar to that in appendix D in ‘Code for Design of Steel Structure’. The results show that effective length factor of non-sway RBS steel frames is very close to that of non-sway conventional steel frames, so the paper suggests that the effective length factor of columns in non-sway RBS frames can be replaced by that of columns in non-sway conventional steel frames; but the effective length factor of sway RBS steel frames is greatly large compared with that of sway conventional steel frames, which should be considered in engineering design.