Abstract: Based on the adhesive fracture test results of new and old concrete of the authors, a toughness model of adhesive interface of new and old concrete is put forward. The characteristic and influence of interfacial micro-crack process of new and old concrete on adhesive fracture toughness is discussed. A banded micro-crack fracture process zone model of adherence of new and old concrete is built up, and the fracture process zone size is obtained using the banded micro-crack fracture process zone model. Combined with the load-strain test result, the influence of fracture process zone on adhesive fracture toughness is modified using the critical fracture toughness.