Through measuring hardness and magnetic behavior of high chromium cast iron（3Cr14Mn4B）,microstructure and hardness change of the cast iron after sub-critical,destabilized and cryogenic treatment were examined to understand the effects of retained austenite and martensite on hardness of the cast iron.The results show that the microstructure of the high chromium cast iron is composed of martensite,a small amount of austenite and（Cr,Fe）7C3 eutectic carbide.In the condition of sub-critical treatment,the hardness of the cast iron is firstly increased and then decreased with increasing in sub-critical treating temperature at air cooling,which is lower than that of one at cryogenic treatment,however,it is higher than that at cryogenic treatment with sub-critical temperature more than 550 ℃.The hardness of the cast iron is slightly increased with destabilized temperature increase,which is higher at cryogenic treatment than that at air cooling.After different heat treatment,hardness of the high chromium cast iron is controlled by martensite content in cast iron matrix and carbide content in martensite phase.The optimized heat treatment process is sub-critical treatment at 550 ℃.