So far most common conversion tables are not including the conversion relation between Surface Rockwell hardness HR15YW and other hardness. It is urgent to find the relation with the HR15YW and other hardness. This paper will focus on the conversion relation between HR15YW and HR15TW through a large number of experiments on three kinds of materials with different hardness and correlation analysis, curvilinear regression analysis. In this paper, there is a strong positive correlation relationship between HR15YW and HR15TW through a lot of experiments and correlation analysis, and get conversion relation between HR15YW and HR15TW, by using curvilinear regression analysis of the least squares estimate principle. The existing conversion tables are supplemented and the HR15YW could be converted other hardness.