Bimodal nano/micro grain structure is prepared in 1020 carbon steels by aluminothermic reaction casting. It is found that the steel consists of a nanocrystalline ferrite matrix and micro-crystalline pearlite phase. The differences in the precipitation behaviors of ferrite by proeutectoid and pearlite transformed from residual austenite are conductive to bimodal microstructure formation. For different substrates of copper and glass, respectively, lamellar space of the pearlite was about 150 and 110 nm, and average grain size of the ferrite matrix is about 35 and 34 nm. Brinell Hardness, compressive and tensile yield strengths, and total elongation are 180 and 162 HB, 400 and 600 MPa, 275 and 230 MPa, 20 and 10%, respectively.