Advanced numerical modeling for cold-formed light gauge steel structures, from manufacturing to the structural response under the applied loading, requires the knowledge of the stress-strain behavior of the material over the full range of tensile strains. Existing stress-strain models for carbon steels are either only capable of accurate predictions over a limited strain range or defined by many material parameters and the values of some material parameters are not available in most of existing design codes. Therefore, a new stress-strain relationship for light gauge carbon steels up to the ultimate strength is required for the advanced numerical modeling and needs to be modeled on the basis of three basic material parameters, the so-called Ramberg-Osgood parameters (the 0.2% proof stress σ , the initial elastic modulus E and the strain-hardening exponent n ). This paper presents such new stress-strain models for light gauge carbon steels, which are able to describe the stress-strain relationship over the full range of tensile strains by using only three basic Ramberg-Osgood parameters. In the present study, the stress-strain data obtained from tensile coupon tests reported in existing literatures have been collected and analyzed, and these tested coupons were cut from both virgin steel sheets and cold-formed steel sections. The new models have been developed by a careful interpretation of these existing experimental data. The accuracy of the proposed models has been demonstrated by comparing their predictions with experimental stress-strain curves.