Abstract: The microstructure,hardness and element distribution of domestic large-size die block of the 3Cr2NiMnMo steel were studied by means of optical microscope,SEM,hardness tester and energy spectrometer.The die blocks before and after high temperature diffusion annealing were compared.The results show that there exist abnormal coarse acicular microstructure in the large-size die block before high temperature diffusion annealing,which was mainly C-rich lower bainite and granular bainite,whose hardness was much higher than the matrix.Element segregation of Cr,C,Mo was observed also.The process of high temperature diffusion annealing could improve the element segregation and eliminate the abnormal microstructure.The microstructure and hardness uniformity of the die block were improved significantly,that could reach the export product standard.