The fractographic analysis of the fracture surface is one solution to determine the kind of failures and predict the fatigue lifetime and resistance of mechanical components under cyclic loading. In the present research, the fracture behavior of the AZ91 magnesium alloy, under stress-controlled high-cycle fatigue loading, was analyzed based on the fatigue striations on the fracture surface. At first, equations and relations were extracted with the Paris crack growth law and the space of the fatigue striations. Then, striation spacing was measured by the experimental results of the high-cycle fatigue (HCF) testing for heat-treated Mg-Al-Zn alloys, containing and non-containing rare earth elements. Finally, constants of the Paris law were calculated and calibrated. Results showed that rare earth elements addition decreased the space between the striations, and subsequently, the fatigue resistance of the AZ91 alloy increased. The reasons are extreme grain size reduction by heat-treating and formation of new Al ₁₁RE ₃ phase by rare earth elements addition. The obtained results of the predicted fatigue lifetime, in comparison to the experimental ones, the scatter band of ±1.5X demonstrated the accuracy of the recommended model by a mean error of 33%.