Fatigue strength curves that are established from fatigue tests provide a basis for the fatigue assessment applying nominal stress approach. In the codes valid for steel structures, like the EC 3, the fatigue strength curves for constant amplitude loading have a knee point in the transition region. The fatigue strength curve beyond this knee point is commonly assumed to be a horizontal asymptote. However, the behaviour of the fatigue strength curve in the area of very high cycles and more importantly the existence of an endurance limit are much discussed. In the case of welded joints the experimental data beyond 107 load cycles is limited due to the possibilities in testing. Testing techniques with high frequencies are necessary to obtain experimental data with very high cycles in a reasonable period of time. In this scope a testing device with approximately 390 Hz operates by alternating current magnets and using resonance amplification, which was developed by a third party. This testing device was investigated and advanced for the application of long term tests reaching 5·108 load cycles. Fatigue tests on axially loaded butt welds with constant amplitude loading are conducted in three test series until very high cycles. The fatigue tests include the area of high and very high cycles. The influence of test frequency and stress ratio is investigated.