Abstract: The quantitative analysis on austenitic grain size and undisolved carbide of M2A1 and M2 high speed steels undergone once and twice quenching indicated that two types of growth, i.e. the normal (homogenious) and abnormal (hetogenious) growth happened upon the same condition of r/f ratio at high temperature depending on initial structure before quenching. A “continious-disco-tinious” recrystallization model and a “merge-migration” growth mechanism had been proposed based on the structure abservations in the beginning of abnormal coasening. Therefore the technical priciples for reducing or eliminating the mixed grain tendency of M2A1 steel was provided.