Abstract: The relation between shape ratio and depth of fatiguecrack on sucker rod is studied using fracture mechanics. The results show that theshape ratio of fatigue cracks with different depth will develop toward astable value approximatively for whatever initial shape. The crackextension test is carried out to determine the asymptotic shape ratio of thefatigue crack, and specimens made of 20CrMo with circular section are used.A linear fitting line is obtained from the experimental data and the shape ratio offatigue crack on sucker rod with diameter of 15.9\,mm is determined to be0.78.