This study aims to determine the effect of plastic deformation percentage variations (0%, 25%, 50% and 75%) in SMAW (Shielded Metal Arc Welding) welding of AISI 1020 on the microstructure and mechanical properties of the material. Welding is carried out with a current of 80A and a voltage of 30 V. The results of macroscopic observations found that the greater the proportion of plastic deformation, the greater the width of the HAZ (Heat Affected Zone). The results of microstructural observations with an optical microscope show that the greater the proportion of plastic deformation, the finer the shape of the ferrite in the BM (base metal) region. On the other hand, in the FGHAZ (Fine Grain Heat Affected Zone) region, the ferrite shape becomes coarser as the proportion of plastic deformation increases. Meanwhile in the CGHAZ (Coarse Grain Heat Affected Zone) region, a ferrite phase is formed at grain boundaries with coarse pearlite for each variation of plastic deformation. The results of the hardness test show that the greater the proportion of plastic deformation, the greater the hardness value in the BM region. In the HAZ and WM regions, the greater the proportion of plastic deformation the lower the hardness value. The results of the tensile test show that the greater the proportion of plastic deformation, result in greater UTS (Ultimate Tensile Strength) and YS (Yield Strength) values.