Weldability of ultra low carbon and nitrogen, low chromium ferrite stainless steel is analysed by using Thermol-cal software and welding metallurgy. Eembrittlement of welding joint is the failure reason during application of ultra low carbon and nitrogen 12% chromium FSS. Comparing welding joint performance of different welding process, Gas Metal Arc Weldinng with high toughness welding material and proper welding heat input is economical and feasible welding process. Controlling growth of ferrite grain is the key to improve toughness of the heat affected zone (HAZ). Presence of titanium carbides or nitrides and the amount of martensite located along ferrite grain intergranular boundaties are very important for toughness of HAZ in low chromium FSS. It was found that the best size of Ti(C/N) grain is 2-5μm and content of martensite is 40%.