It is well known that cooling rate (Rc), undercooling (ΔT) and microstructure in solidification are not independent, but the simple relationships are rarely reported. We aim to present the relation of Rc, ΔT and microstructure through solidification of iron melt by molecular dynamics simulation and classical nucleation theory (CNT). The results show that if Rc⩽1011.97K/s, the iron melt will solidify as bcc crystal structure; if 1011.98K/s⩽Rc⩽1012.3K/s, the mixed structure of crystalline and glass will be obtained; if Rc>1012.3K/s, the iron melt will solidify as amorphous structure. From the CNT, a relationship between Rc and ΔT can be deduced, by which, the critical cooling rate for forming glass is predicted as 1011.43±1K/s that is near the simulation result (Rc>1011.97K/s).