The effects of preheating, post weld heating and arc welding parameters on the microstructures and mechanical properties of SAILMA 300 HI steel welds in comparison to normal welds were experimentally determined. The results show that when preheating and/or Post Weld heat temperature increases the yield strength, ultimate tensile strength, and hardness of welds slightly decreases, on the other hand percentage elongation, percentage reduction in area and toughness increases as compared to normal welds.The combined condition of pre- and post- heating of welds showed best improvement in toughness. The effect of heat inputs (function of current, voltage and travel speed) on mechanical properties also show same pattern as preheating.Micro structure studies reveal that when preheating and post heating temperatures and heat inputs are increased, tempering of brittle phases and grain coarsening occurs, as compared to normal welds.This combination provides maximum volume fraction of acicular ferrite in welds which is desirable because it provided a good toughness and tensile strength to the welds.