Abstract: The strength（σ_b）, impact toughness(A_kv), microstructure and grain fineness of two batches of steel 35CrNi3MoVA are studied regarding variations in the content of Si & Mn. The effects of Si & Mn's variation on toughness is discussed, and the related curve between K_IC and A_kv analyzed. Test results showed the effect of impurity elements on the toughness and fractography, are more marked on those of other properties. In addition, the value of A_kv is elevated, the fracture tearing edge is higher and the tearing dimple is deeper on the condition of higher Mn, lower Si in this steel.