The sustained load tensile test is done to evaluate the delayed fracture resistance of both class 13.9 ADF1 steel bolts with DAC treatment and class 10.9 42CrMo steel bolts with phosphate treatment in air and in 3.5% NaCI solution. The fracture surfaces are examined with the help of scanning electron microscopy (SEM). The results show that the delayed fracture strength of class 13.9 bolts is higher than that of class 10.9 bolts and its fractured appearance exhibits a mixed mode failure process and transition from trans-granular feature in air, and both inter-granular fracture and trans-granular fracture in 3.5% NaCI solution.