Following the rolling and heat treatment processes, TMT bars have a tempered martensite outer with a ferrite pearlite core. The martensite zone is at least twice the tensile strength of the pearlite. This research was aimed at establishing the regression between the thickness of the outer layer and the overall ultimate strength of the steel bar. Thirty bars of TMT were selected and subjected to tensile testing. Polished sections of these bars were subjected to surface micro-hardness tests. A plot of hardness against the radial displacement along the section of the bar was made to enable determination of the 50% position. The thickness of the martensite zone up to the 50% position was made against the ultimate strength of the bar in each case. A direct proportionality in y=438.36 X +104.3  resulted at 0.9278 determination to show that the martensite ring thickness is directly proportional to the ultimate strength of the thermo mechanically treated bar.