Abstract: Compress experiments on 5139 samples of nine grades of carbon steel which were collected from 1700 hot strip mill in WISC were made by a cam plastometer with constant strain rate, the flow stresses of carbon steels measured. Seven appropriate types of flow stress mathematical models were chosen to make regression analyses of these experimental data. A new - mathematical model was obtained in wfich the influences of main chemical elements in carbon steel and deformation conditions were fully considered. The structure and effect of new mathematical model are better than that of being used and it has been successfully used for on-line production control of 1700 hot strip mill in WISC.