In this paper, the governing equations of the outrigger-braced structure are formulated by considering the influence of core-wall width. The axial forces in the columns are considered as unknowns and the compatibility of the column's axial deformation is considered in formulating the governing equations. Then, the governing equations for the optimum locations of outriggers are formulated by minimizing the top drift of central core wall. After that, the influence of core-wall width on the optimum outrigger locations, the drift-reduction efficiency and moment-reduction efficiency is investigated numerically. Some results are presented for the reference in the preliminary design of the outrigger-braced structures.