In order to improve the accuracy of forming analysis for aluminum alloys, it is important to determine precise flow stress equation for aluminum alloys. In case of 5000 series aluminum alloys, formability is difficult to predict due to non-uniform deformation such as dynamic strain aging and stretcher-strain. To identify flow stress curve of A5182-O for wider strain range, the methods using rolling-tensile test and Vickers hardness test was investigated and compared with conventional regression curve obtained by uniaxial tensile test. The three methods were assessed in prediction of neck formation. It is found that stress-strain relationship without dynamic strain aging effect is required for precise prediction of uniform elongation. Plastic instability condition was predicted more precisely by either rolling-tensile test or Vickers hardness test than by conventional tensile test.