DIN 34CrNiMo6 is a high-strength low-alloy steel, similar to SAE 4340. As the name says, this term is given to steels which contain low alloy level and high-strength limit. The advantage this steel has in relation to others high-strength steels is that it has also a high fracture resistance. This steel has good hardenability too, which means, high depth intrusion of martensitic transformation over its axis, in a way that not only the external part become hard, but also the innermost parts. The main alloy elements are Cr, Ni and Mo, while its two last digits (40) refer to the percentage of carbon that the steel contains, in hundredths. Because this type of steel is very used in mechanical components, such as axis and crankshaft, it is important to study fatigue in this material to predict its service life and avoid catastrophic failures. This study was made based on the S-N methodology to determine the fatigue limit of DIN 34CrNiMo6 steel, admitting its infinite life equal to 107 cycles. For this purpose, rotating beam fatigue tests were made on standard cylindrical specimens submitted to totally reversible cyclical loads. With the results, it is possible to adjust a S-N curve to experimental points befitting to Wöhler s curve and determine the endurance limit of the structural steel DIN 34CrNiMo6.