In order to explore the effect of rare earth (RE) element on the strength and toughness of steel, mixed La+Ce elements have been incorporated into Nb-alloyed steel at the content of La+Ce: 36ppm, La+Ce: 44ppm, La+Ce: 51ppm and reference sample (no RE). The mechical properties and microstructure of the materials were analyzed by using tensile test, low temperature impact test and SEM observation coupled with EDS. The results indicated that the reduction of Nb and the increase of the La+Ce influenced the mechanical performance of Fe-0.07%C-0.025%Nb-x%(La+Ce) component system, with the?increase of RE, dimples were enlarged and deepened, and the yield strength and tensile strength of Nb-alloyed structural steels tended to increase. meanwhile the elongation did not show a significant reduction with the increase of RE content. These results demonstrates that the mixed La+Ce rare earth compound is promising to replace the Nb element to improve the mechanical performance of the product. The comparison of fracture morphology and absorbed impact energy of different kinds of hot rolled steel belt show that the low-temperature impact toughness maximized at -40℃ and -60℃ when the content of La+Ce rare earth compounds was 36ppm, due to the fact that inclusions were modified to form precipitated LaCeO2S particles which delayed the crack growth.