Abstract: The characteristic crystal model is applied to the study of phase stability and thermodynamics in substitutional bcc-based Ta-W alloys. The basic principle and method of alloy thermodynamics which is described by the characteristic crystal model is introduced in detail. The thermodynamic properties of the Ta-W system are evaluated to describe the Gibbs energies of various phases including both disordered and ordered phases. The phase diagram of Ta-W is assessed and predicted. The results are in good agreement with experiments and better than that of first principle.