It is significant to predict ductile fracture initiation in plastic forming of metal materials. Tube hidroforming (THF) is attracting general attention.THFprocess requires the control of internal liquid pressure and axial compression. In this case a ductile fracture may occur under stress triaxiality lower than that for uniaxial tension loading. In this report, combined longitudinal tensile and compression to thickness direction tests using thin plate specimens inA5083-Oaluminum alloy to obtain low stress triaxiality were carried out. On the basis of observations of ductile fracture processes it was founded that ductile fracture was shear type fracture controlled by a nucleation of micro-voids in this test. It is seemed that the local inclusion-matrix interfacial stress was constant independently of any stress ratio of tension and compression. Additionally, THFtests for SPFH590 tubes were performed. It was confirmed that the critical interfacial stress obtained using FE analysis forTHFtest was constant.