Abstract: Effective flange width is one of the critical parameters when checking the flexural strength of end of composite frame beams. The existing formulae for effective flange width have inappropriate load case, inappropriate equivalence rule, and thus cannot reasonably consider the non-symmetry of steel sections. Therefore, the main focus of this paper is to study the effective flange width of end of circular-section-column composite frame beams with non-symmetrical steel section under lateral load at ultimate limit state. Elaborate numerical models are established using MSC.MARC (2018) and the critical parameters that significantly affect effective flange width are identified by univariate analysis. An extensive numerical analysis is carried out with varied critical parameters within reasonable ranges. The formulae for effective flange width of the composite frame beams with non-symmetrical steel section under lateral load are put forward by keeping the basic framework which is established in previous research, and modifying some parameters in the formulae for symmetrical steel section through regression of numerical results.