The influence of rare earths(RE) and Mg compound modification on eutectic carbide morphology and matrix of high speed steel(HSS) roll was investigated. The results show that the matrix can be refined by the RE-Mg compound modifier, the eutectic carbides are inclined to turn into small lump, whose distribution is homogeneous. After modification, the heat conductivity coefficient of HSS is increased and heat expansion coefficient is decreased. Moreover, RE-Mg modification is able to increase the cycles of thermal fatigue crack initiation and decrease the propagation rate. After cycling 700 times at the condition of heating at 600 degrees C and cooling in 25 degrees C water, the length of thermal fatigue crack is lessened by 41.12%.