Fatigue behaviour of thin-walled tubular specimens subjected to tension and torsion loading as well as in-phase and out-of-phase combined tension-torsion loading was investigated using the mild steel Ck 15. In addition to lifetime and deformation behaviour the development of microcracks was observed. Lifetime and deformation behaviour of the mild steel Ck 15 under multiaxial loadings can be described by the von Mises criterion. A small out-of-phase hardening was observed. Cracks occur in the plane of maximum shear-stress amplitude. The cracks grow by linking with other cracks.