The effects of arc welding on the fatigue strengths of three HSLA steels and 1006 mild steel for joint and loading configurations typical of automotive applications have been investigated. It was found that the 107 cycle fatigue limit stresses of smooth specimens varied according to the tensile strengths while the fatigue characteristics of all four steels after welding were nearly identical in the 5 × 105 – 107 cycle range. Long life fatigue data from both the smooth and welded specimens were used to determine the applicability of Peterson’s Equation. Predicted Kf’s were 13 to 17% larger for SAE-950, SAE-980 and 1006 steels, but was 22% less for GM-980X, compared to the experimental results. A fatigue crack initiation study was conducted at the stress range that would result in a total life of 105 cycles for each alloy. Initiation of cracks .025 cm deep occurred at approximately 88% of total life for the three HSLA steels and 55% of total life for 1006 mild steel.