Abstract: The strength and toughness of ultra-fine grained steel are raised a great deal compared to the normal grain steel.Weld ability of ul-tra-fine grained steel is important for application of the steel.In this paper welding and thermal simulation are conducted to study the effect of welding heat input on the micro structures and mechanical properties of the steel.Experimenal results indicate that the tendency of austenite grain growth of ultra-fine grained steel is almost the same as normal grain steel.There is a softened zone between heat affected zone and base metal be-cause of recrystallization.For the steel plate thinner than 3 mm,the absorption energy of crack propagating in coarse grain HAZ is higher than that of base metal.For the steel plate thicker than 5 mm,the toughness of coarse grain HAZ is nmch lower than that of base metal.The mechanism that affects the toughness of coarse grain HAZ is studied.