Abstract: In this work the effect of alloying elements molybdenum and vanadium on the low temperature temper embrittlement of Si-Mn steels has been investigated by means of impact test, fracture toughness test, fractography and ion probe micro-analysis. The results of the investigation showed that Si-Mn steels have obvious low temperature temper embrittlement when molybdenum is absent. This embrittlement occurs at 360℃. The addition of about 0.3% molybdenum to this steel can greatly reduce or completely suppress the 360℃ embrittlement. Vanadium ranging from 0.05 to 0.33% has no significant influence on the 360℃ embrittlement. The behaviour of the effect of molybdenum and vanadium on fracture toughness is analogous to that of the effect on impact toughness. Based on the results of ion probe micro-analysis it has been shown that the 360℃ embrittlement of Si-Mn steels is associated with the segregation of phosphorus on the grain boundaries of prior austenite.