Abstract: A practical model to estimate fracture toughness of concrete was developed by utilizing the equilibrium of forces at the cracked section in unstable fracture state of compact tension specimen, where the softening feature of concrete was taken into account. With Young’s modulus and the stress at the effective crack tip as two only prerequisites, the model can predict the maximum load and its corresponding effective crack length, and in turn the fracture toughness of concrete specimen under compact tension. Compared with test data available, a favorable agreement was observed for the maximum load and its corresponding crack mouth opening displacement. Based on the model presented, which is simple in theory and accurate in practice, size effect on fracture toughness was investigated. The results indicate that the fracture toughness from the proposed model exhibits little size effect for specimen of normal size in laboratory, and could be regarded as material constant, which could be used to determine whether a crack will propagate.