Abstract: Based on the crack propagation criterion for the mode I crack， a practical analytical method is developed for determining the crack extension resistance curves of concrete. In this method， the crack extension resistance is composed of the initial fracture toughness KiniIc and the resistance caused by the cohesive force， equaling the driving force of the crack extension. It is concluded that as long as the initial fracture toughness KiniIc， the elastic modulus E， the tensile strength ft and the fracture energy GF of concrete are obtained，the KR extension resistance curves can be calculated by using this method without testing the load-crack mouth opening displacement curves. Further， the practical analytical method is applied to calculate the KR extension resistance curves，the fracture process zone lengths and the cohesive stress distribution for concrete with varied strength grades from C20 to C120，different initial crack length/depth ratios and different depths under the three-point bending conditions. Based on the numerical analysis results， it is found that the KR extension resistance curves increase with the increase of the strength grades，but the growth amplitude decreases，and when the strength grades is larger than C60， the growth amplitude will tend to be stable； the KR extension resistance curves are dependent of the initial crack length and specimen depth，and when the ligament is long enough， the crack extension resistance increases and then tends to be stable；the maximum lengths of the complete fracture process zone decrease with the increase of concrete strength grades；and the specimen size has remarkable impacts upon the fracture process zone length.