Abstract: The effect of different residual element contents of tin and antimony and rare earth element lanthanum on the impact toughness of 34CrNi3Mo steel with low oxygen and sulfur contents was investigated with JB-30B impact test machine. SEM and EDS were used to study lanthanum effect on the fracture mode of the steel with different tin and antimony contents and the rare earth inclusions. The results show that rare earth metal lanthanum may decrease remarkably the harm of residual elements on the toughness of the steel and combine with residual elements in the steel to form La-Sb intermetallic compound and a little amount of La-Sn intermetallic compound, presenting orderly distribution.