Abstract: The arch three point bending specimen was first suggested as the one of fracture toughness specimens by comrade Cheng Chi of our country. In order to perfect the mechanical calibration of this specimen, we determine the stress-intensity factors K_I with photoelastic technique. Data management are conducted by application of Taylor Series Correction Method (TSCM) and Schroedl-Smith Method. In fitting the exper-imental data with cubic curve, the formulae of regression coefficient are derived in this paper. The photoelastic results are in good agreement with the results of compliance calibration method.