By reducing ferrite-grain size from 25 v m to 7 11 m, the yield strength of a carbon steel is doubled from 200 MPa to 400 MPa and the ultra-fine grained steel also has good low temperature toughness. During welding its microstructure is changed and the mechanical properties are affected. Thermal simulation, gas metal-arc welding and shielded metal-arc welding are used to evaluate the effect of welding thermal cycle on the steel. The effects of cooling time between 800degreesC and 500degreesC and the time above 800degreesC on the transformation temperature and microstructure are analyzed. Experiment results indicate that the low temperature toughness of coarse-grain HAZ is about half of the base metal. The factors that influence the toughness of HAZ and welded joint are reviewed. There are softened zones between HAZ and base metal. If small heat input is used, the width of softened zone is narrow, the degree of softening is small and the tensile strength is not affected. The welded joint has good ductility.