The cold work from the manufacturing process of cold-formed sections causes a significant change in the mechanical behaviour and enhances the strength of the material. The enhanced strength of the material in corners (the so-called corner material) has been traditionally determined using semi-empirical models. However, these semi-empirical models for the corner material are only applicable to the prediction of the enhanced yield strength (or 0.2% proof stress), but are neither capable of predicting the stress-strain behaviour of the corner material nor able to account for the difference in the mechanical behaviour of the corner material under tension and compression. This paper presents the results of a study on the accurate prediction of the mechanical properties of both virgin stainless sheet materials and corner materials of cold-formed stainless steel sections, which overcomes these difficulties. This paper first presents a new stress-strain model for stainless steels, which is capable of accurate predictions over the full ranges of both tensile and compressive strains. The new stress-strain model is defined using the three basic Ramberg-Osgood parameters and is based on a careful interpretation of existing experimental data. In the second part of the paper, a finite element method is then presented for predicting both tensile and compressive stress-strain behaviours of the corner material in cold-formed stainless steel sections. In this method, the effect of cold working on the stress-strain behaviour of the corner material is account for by means of a numerical simulation of the cold-forming of corners, with the resulting residual stresses and equivalent plastic strains specified as the initial state in a subsequent finite element simulation of corner coupon tests. The accuracies of both the new stress-strain model and the finite element method are demonstrated by comparing its predictions with experimental stress-strain curves. The proposed finite element method together with the new stress-strain model can be applied in the future in a development of a direct relationship between the virgin material properties and the corner material properties.