The　effect　of　nitrogen　on　crystal　grain　for　metastabl　austenitic　manganese　steel　was　studied.　The　result　indicated　that　the　attachment　of　nitrogen　element　in　the　interface　between　the　crystal　and　liquid　steel　will　influence　the　formation　mechanism　of　crystal,　that　the　existing　of　nitrogen　element　hinder　the　diffusion　of　atoms,　increase　the　free　energy　of　critical　nucleation,　raise　homogeneous　nucleation　rate　of　crystal　and　lower　the　growing　velocity　of　crystal　accordingly,　and　refine　the　crystal　grain　finally　for　casting　steel.　And　the　effect　degree　of　nitrogen　will　be　influenced　by　the　specialty　of　nitrogen　on　critical　micelle　concentration.　During　heat　treatment,　because　of　covalent　bond　between　nitrogen　atom　and　iron　atom　and　manganese　atom,　the　existing　of　nitrogen　element　in　the　boundary　of　crystal　or　the　attachment　of　nitrogen　element　on　the　face　of　crystal　will　hinder　the　diffusion　of　iron　atoms　and　manganese　atoms,　so　that　lower　the　growing　velocity　of　crystal　and　refine　the　crystal　grain　of　steel　finally　after　heat　treatment.