Abstract: The geometrical parameters of a screw bolt are important factors that affect the fatigue strength of the bolted joints. Therefore, the author has made selective measurements and statistical treatments from among a large number of samples of five models of screw bolts by way of project-analogue method. The analysis result has indicated that the minor diameter d₁, the round radius at the bottom of the scr-ewr and the round radius under the head of the bolt R are distributed according to the normal. From the above conclusion, We have determind their distributive parameters. Hence, a sound basis is provided for the reliability design of the fatigue strength of the bolted joints, the reasonable consideration of its geometrical effect and the formulation of the reliability index of the screw bolt.