FactSage software and SEM + EDS were introduced to determine the composition of nucleation inclusion in Ce-treated Ti–Mg deoxidized C–Mn steel with Ce content ranging from 0 to 0.036 wt%. The nucleation mechanism of inclusion in tested steels was investigated simultaneously. The experimental results showed that the composition of inclusion in the Ti–Mg complex deoxidized steel changed from MgO + MgAl2O4/MgTi2O4 + MnS to MgO + MgAl2O4/MgTi2O4 + CeAlO3/Ce2O2S + MnS after Ce treatment, while there was no significant change in the content of Ti(C, N). The mass fraction of Ce2O2S was markedly increased with the increase of Ce. The addition of Ce enhanced the probability of nucleation of acicular ferrite on inclusion. The nucleation mechanism of acicular ferrite induced by inclusion containing Ce was manganese-depleted zone (MDZ) based on the line scanning results and the low lattice mismatch between Ce2O2S and ɑ-Fe.