Uncertainty pervades our everyday lives. In almost every domain of our work we come face to face with uncertainty, which is classically probabilistic in nature. The advancement of technology made it imperative to measure the physical properties of material, machine components with more accuracy. The uncertainty is a parameter associated with the result of a measurement that characterizes the dispersion of the values that could reasonably attributed to the measurement.Thermo-mechanically Treated (TMT) Steel Bars are now universally accepted material for Reinforced Concrete (RCC) Structure. These reinforced steel bars are now being manufactured using thermo-mechanical treatment, in order to achieve the best possible combination of strength, ductility and bonding characteristics. The production of TMT Bar can be broadly classified into following steps: Steel Melting, Billet casting, Billet Reheating (if not directly casted Billet fed into rolling mill). Total Rolling procedures, Quenching. Self-tampering and aging. Consequently, there is a chance of measurement of uncertainties (MOU) in the final enducing properties of TMT Bar.In these studies, an attempt has been made to evaluate numerically, the measurement of Uncertainties of Tensile Test parameters of TMT Bars. These steel TMT Bars have been taken from same Billet stock, in order to fix-up the chemical composition of Steel Billet as constant. Significant variations in MOU (±%) observed for each test parameters (YS, UTS, % elongation) for the different sizes (8, 10, 16, 20, 26, 28, 32 mm). This information will be an important observation as far as acceptance criteria and use of steel TMT Bar, for Quality Control & Assurance (QCA) purposes.