The failure analysis of a rocker arm shaft for a 4-cylinder SOHC engine is presented. Fracture accidents occur in the interface between the rocker arm shaft and the engine block. These fractures are responsible for bolt tightening. The failure boundary condition is estimated by using an orthogonal array and ANOVA. Stress range is obtained by fatigue striation spacing and fracture mechanical simplification. Minimum and maximum stresses are predicted by FEA for the failure boundary condition. This study shows that the stress conditions for a fracture surface with fatigue striation can be determined by FEA and SEM.