In this study, AISI 4140 steel couples 10 mm in thickness were joined using manual gas metal arc (GMAW) and synergic controlled pulsed gas metal arc (GMAW-P) welding methods. Interfacial images of welded samples were investigated using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) methods. Impact and microhardness tests were performed to determine the mechanical properties of the joined parts. In addition, the fracture surface of the broken joint with the impact test was examined using SEM. The AISI 4140 welded joints obtained by the GMAW-P method showed higher impact strength, less grain growth and narrower heat affected zone (HAZ) than by using the GMAW method. It is thought that this is caused by lower heat input.