Abstract: Using the three-column subassemblage model, the governing equations for determining the effective length factor (μ-factor) of columns in sway composite frames with semi-rigid connections are derived, which consider the effects of the nonlinear moment-rotation characteristics of beam-to-column connections and composite action of slab. The effects of various beam end conditions and column far end conditions on μ-factor are also considered. Furthermore, accuracies of GB50017-2003 specified appendix D solution are also assessed. Finally, the influences of load value and the nonlinear moment-rotation characteristics on μ-factor are numerically studied by using two examples of portal composite frame and three-story-two-span composite frame. It was found that using GB50017 specified appendix D, the μ-factors in the subassemblage models with various boundaries provide a conservative design of columns, and the μ-factor is significantly affected by the initial connection stiffness. The proposed formulas can be used for the structural design in a practical engineering.