Abstract: For two-dimensional plane stress problem, the relationship between macroscopic damage variable and the averaged microcrack density parameter for concrete under biaxial tension stress is presented based on the equivalence hypothesis of energy dissipation. Mixed mode microcrack stability criterion is used in this paper. Results show that the macroscopic damage value at failure under biaxial tension stress is larger than that under uniaxial tension stress, and it approximately increases proportionally with the increase of lateral tension stress. Comparison with micromechanical results has been made. The resultant effective elastic moduli coincide well with the results of self-consistent method, and approximately agree with the results of non-interacting method.