The effect of tensile mean stress on the fatigue limit of carbon steel was investigated. Annealed 0.1 % and 0.5 % carbon steel specimens were subjected to monotonic tension to produce prestrain. Fatigue tests were performed using a servohydraulic fatigue test machine. The experimental results showed that tensile mean stress degraded the fatigue limit. However, the tensile mean stress did not affect the decrease in the surface hardness as a result of fatigue loading for the specimen without a defect. In the case of the specimen with a defect, the decrease in the hardness was small and did not depend on the mean stress or stress amplitude. Formulae for predicting the fatigue limit of prestrained carbon steel specimens under tensile mean stress were proposed. The results calculated using the formulae showed good agreement with the experimental results. Copyright © 2008 by ASTM International.