We present a eutectic phase-field model, developed by extending the interface field method (Steinbach and Pezzolla, Physica D 134 (1999) 385) under a condition that coexisting phases at a given point have an equal chemical potential difference between solute atom and solvent atom. Also an anisotropic eutectic phase-field equation is derived explicitly. The equilibrium interface geometries, calculated using the isotropic and anisotropic models, at a triple junction under a thermal gradient are in good agreement with the exact solutions, indicating the maintenance of mechanical equilibrium at the junction. The model successfully reproduces a variety of eutectic lamellar patterns observed in experiments on directional solidification of thin film CBr4-C2Cl6 organic alloys, under the real experimental conditions without any fitting parameters. Most of the characteristics of the pattern change with lamellar spacing in both eutectic and hypereutectic alloys are in good agreement with experimental observations. (C) 2003 Elsevier B.V. All rights reserved.