The Grain Size Strengthening is part of Strengthening Mechanisms courses. The Strengthening Mechanisms is part of the Ferrous Metallurgy set of courses. In this course, the MLI (Mean Linear Interception) will be evaluated in relationship with the grain size. Then, the ASTM method to calculate grain size and the relationship between tensile strength and grain size will be introduced. After that, the relationship between the grain size and mechanical properties will be showed. Finally, the Charpy impact transition temperature (ITT) will be introduced and Mintz, Baird and Cochrane results will be studied.