this paper analyzed the relief influence on the thickness of engine cranks web at rounded stress
and solved the stresses distribution of three different structural crankshafts under the same condition. Through analyzing the result
we can draw a conclusion that it is an effective way to increase the thickness of crank web than to increase the shaft diameter by the way which improves the rounded stress of the crankshaft from structural. The result provides the theory for an engine crankshaft design.