This paper proposes a multi-objective optimization design methodology for coupled inductors in multiphase Buck converter. Efficiency, transient performance and size are the three dominant factors for design consideration. According to the analysis in this paper, there are tradeoffs among these three factors. Taking into account current ripple, transient equivalent inductance and saturation flux density, a design space can be determined by several intuitive constraints. An optimal design point of coupled inductors can be selected from the design space to achieve optimum converter performance. The proposed design methodology is verified through a six-phase multiphase Buck converter.