With the dental implant system, the FEA (Finite Element Analysis) allows researchers to predict stress distribution in the contact area of the implant with cortical bone and around the apex of the implant in the trabecular bone, and then, predicting the fatigue strength of implant which is very important in design and manufacture because it is almost impossible to correct after the implant was inserted. In this paper, the fatigue strengths of three implant systems are estimated. The stress fields in implants are calculated by elastic-plastic finite element analysis, and the equivalent fatigue stress, considering the contact and preload stretching of a screw by torque for tightening an abutment, is obtained. Fatigue life, which is affected by the contact in the screwed interface and pretension in the screw, is then determined. To evaluate the reliability of the calculated fatigue strength, fatigue test is performed. The calculated and experimental results showed good agreement. The fatigue strength in the ITI was higher than that of other types.