Abstract: On the basis of high strength low alloy steel flux cored wire which was designed by authors, the effects of micro-alloying elements nickel and niobium on mechanical properties of welded joint were researched by tensile test, charpy V-notch test, et al. Combined with metallographic observation and scanning electron microscopy analysis and chemical analysis, the mechanism about the effect of micro-alloying nickel and niobium elements on mechanical properties of welded joint was reasonable explained from the microstructure and chemical composition. The results show that the micro-alloying nickel element in the flux cored wire plays the role of increasing strength of welded joint by increasing the contents of bainite and martensite in the weld metal, and improve impact toughness of welded joint at low temperature through promoting the nucleation and growth of acicular ferrite. The micro-alloying niobium elements in the flux cored wire has the same effect with the micro-alloying nickel element that is beneficial to promote the nucleation and growth of acicular ferrite, meanwhile it can enhance the tensile strength of welded joint by refining grain and precipitation strengthening.