Abstract: A kind of medium-carbon steel with microstructure consisting of graphite and ferrite was prepared by method of increasing graphite nucleus.Microstructure of the steel was observed using OM,SEM and TEM,mechanical properties were tested using universal material testing machine,and machinability was tested using lathe.The results show that the precipitation and growth of the graphites were carried out based on the heterogeneous nuclei BN.The graphites homogeneously distributed in grains and at grain boundaries.Size of the graphite was less than 10 μm.The yield ratio of the steel was low(0.59-0.63) and the relative machinability was high (Kν=5.424 1).The tested steel display higher machinability and thus can satisfy the requirement of high speed machining.