In recent years, the usage of dual phase (DP) steels, transformation induced plasticity (TRIP) steels and boron steels in some auto body parts has become a necessity because of their strength and lightweight. Resistance spot welding (RSW) being a process were steel is heated and cooled in a very short period of time, the resulting weld nugget is generally fully martensitic, especially in the case of DP, TRIP and boron steels but that also holds for plain carbon steels as AISI 1010 grade which is extensively used in auto body inner parts. Martensite in is turn must be avoided as much as possible when welding steel because it is the principal source of brittleness. Thus, this work aims in finding a mean to reduce martensite fraction and increase phase diversity in weld nugget. The prediction of phase transformation during RSW has been done. Simulations have been performed for 2 mm AISI 1010 sheets and results show that the application of post weld heat treatment leads to the reduction of martensite fraction, and formation of ferrite and bainite in the nugget. Welding experiments have been done in parallel and experimental weld nugget geometry is in good agreement with simulation results.