The experimental investigation has been made to determine the effect of crystallographic boundaries and chemical composition on the strength and hardness using mild steels of SAE 1016 and 1022. Through heat treatments and quantitative analysis, different microstructures varying in grain size from 4.4.mu.m to 23.2.mu.m and in Fe$_{3}$C particle spacing from 0.56.mu.m to 2.46.mu.m were obtained. Mechanical properties of yield strength, tensile strength and micro vickers hardness were measured and the relationship between these properties and microstructures were obtained. Some of the conclusions are as follows; When mild steels are strengthened by both grain size hardening and Fe$_{3}$C precipitation hardening mechanisms;