Abstract: Based on the concept of cohesive crack and taking initial fracture toughness as a fracture initiation and propagation criteria, the calculation method of a concrete fracture process zone length is developed. The fracture process zone length in the Mode Ⅰ Crack with different initial crack lengths and fracture toughness are calculated and the results are verified by previous experiment data of mass concrete. Furthermore, the affecting factors of fracture process zone lengths are analyzed and the results show that the fracture process zone length gradually increases with initial crack length and decreases with initial fracture toughness.