Abstract: 40Cr steel was normalized at 830, 890 and 950 ℃ using a thermal simulation tester, to obtain ferrite and pearlite structures with different grain sizes. Then the samples were heated to 850 ℃ and kept for 10 min followed by quenching at different cooling rates. The microstructure and hardness variations of samples with different grain sizes were compared after quenching. The results showed that with the increase of grain size before quenching, the content of martensite structure in 40Cr steel sample after quenching increased, while the content of bainite decreased. Meanwhile, the hardness was improved. The larger the grain size of 40Cr steel before quenching was, the better the hardenability was.