Welding of TRIP steels are one of the technical challenges in the successful application of AHSS in chassis structures. Gas Metal Arc Welding (GMAW) is a common welding process used in the automotive industry, for joining mild steels. TRIP steel; however, do not offer the same ease of welding, and process control welding parameters is more critical. The welding parameters window represents the range of acceptable process parameters, primarily control of heat inputs, to obtain an acceptable weld. As a result, the effects of the heat input variations are greater and TRIP steel has a narrower welding parameters window in which acceptable welds can be made. Mechanical properties of the lap joints of TRIP 780 steel 2.8 mm thickness was analyzed, fusion welds were evaluated using fatigue tests on these joints for different heat inputs. Fatigue testing was conducted under a different number of nominal stress ranges to obtain the S/N curves of the weld joints.