Abstract: This paper proposes a method for quantitativetesting of plastic deformation and damage in steel members during the process ofloading. The Diffuse-Reflectance Method was used to obtain varied images of surface roughness of steelspecimen during the process of stretching. With a further treatment ofthese images, the average gray value variations can be obtained to reflect the steel specimen surfaceroughness, which will help us to find out the sensitive parameter relevantto plastic deformation and damage status in the steel member under loading. Theresults show that, based on Diffuse-Reflectance Method and imageprocessing, the average gray value variations of steel specimen's surfaceroughness during the loading process could be used to detect the start ofplastic deformation and to describe the damage degree quantitatively in thelate stage of plastic deformation of metal components or structures.