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It is well known that fretting fatigue is affected by various factors such ascontact pressure, relative slip, contact length, specimen size, loading type and so on.Thereason why these factors affect fretting fatigue was investigated. The stress distributionnear the contact edge was measured using a small stress concentration gage. It was shownthat the stress concentration varied significantly depending on these factors. It was foundthat the S-N curve was expressed uniquely on the basis of the local stress at the contactedge. The meaning of the local stress in fretting fatigue was also studied based on thefracture mechanics. The crack analysis showed that the fatigue limit condition could beevaluated based on the threshold condition for short fatigue crack and the ÄK behavior ofsmall fretting fatigue crack was uniquely determined by the local stress.