Abstract: 40Cr steel was researched as the common material used for automobile crankshaft.TEM was usedto observe the change of microstructure, and to find the relationship between the change of microstructure andmechanical fatigue.The research results showed that, in the same stress level, with the fatigue cycle numberincreased, the dislocations in the 40Cr initiated from the grain boundaries, extended into the grain.And themorphology of the dislocation became to sub grains from bands.The dislocation density increasing caused bymechanical fatigue had liner relationship with fatigue cycle number.The relationship was: ρ=2.88×108 N +5.271 2×108.