Corrosion ratio is an important index to study the mechanical deteriorates of the steel bars, which has a significant effect to evaluate the residual bearing capacity of reinforced concrete structures. To investigate the mechanical properties of the corroded steel bars, Strain energy loss as corrosion ratio is firstly proposed. Tensile test are conducted on ribbed and plain steels, which are corroded by acceleration corrosion method. Comparing with the weight loss and cross-section loss to describe the effect of corrosion of reinforcing bar, the strain energy loss of reinforcing bars is calculated by Simpson quadrature. Results from this paper and other researchers’ test suggest that the strain energy loss may be a better parameter than weight loss or section loss which to assess the corroded steel bars.