The　effects　of　RE-Ti　on　the　as-cast　microstructure,　heat　treatment　microstructure　and　mechanical　properties　of　high　vanadium　high　speed　steel　have　been　investigated.　Results　show　that　when　suitable　modifier　is　added　to　high　vanadium　high　speed　steel,　the　austenite　and　eutectic　carbides　are　changed.　The　austenite　grains　and　coarse　eutectic　structure　are　refined,　and　flake　carbides　in　eutectic　structure　become　short　and　thinner.　After　heat　treatment,　most　of　eutectic　carbides　are　spheroidized　and　distributed　uniformly.　The　alloying　elements　in　the　modifier　can　form　uncontinuous　hard　carbide　within　the　crystal　grains　or　along　their　boundary.　Thus　they　can　refine　the　microstructure　and　improve　the　hardness　of　the　high　vanadium　high　speed　steel.　The　mechanism　that　modifier　improves　the　structure　and　properties　of　high　vanadium　high　speed　steel　is　analyzed.