A phase-field model is presented for studying the solidification of multicomponent alloys. which can consider the different diffusion mechanisms between the substitutional and the interstitial solutes appropriately. By defining the interfacial region as a mixture of solid and liquid with the same inter-diffusion potential and determining and matching the phase-field parameters to the alloy properties at a thin-interface limit condition, the limit of interface thickness due to the contribution of the chemical free energy density in the interfacial region is remarkably relieved and a large-scale calculation domain is permitted. One-dimensional numerical calculations for the solidification of an dilute Fe-Mn-C alloy system demonstrate that the model could provide a good description for the equilibrium and kinetic properties for isothermal solidification of multicomponent alloys. Two-dimensional computations for the evolution of large-scale dendritic microstructures is performed and solute distributions in both the solid and liquid phase and the dendritic shapes are compared at different temperatures. (C) 2004 Elsevier B.V. All rights reserved.