In this paper, in order to identify the formation mechanism of duplex (Ca,Mn)S inclusions in steel, based on the two heats ofcommercial Ca-treated resulfurized steel, the characteristics of duplex (Ca,Mn)S inclusions in bars, blooms and CaO-Al2O3oxides in molten steel were observed and analyzed. The results indicate that there are three types of duplex (Ca,Mn)S inclusionsin steel. The first type with over 20% Ca in (Ca,Mn)S is named as “Type-C”, the second with 4–20% Ca in (Ca,Mn)Sis named as “Type-MC” and the third with below 4% Ca in (Ca,Mn)S is named as “Type-M”. Their core oxides are mainlyCa-Mg–Al-O oxides. The aspect ratios of duplex (Ca,Mn)S inclusions in bars decrease as Ca content in (Ca,Mn)S increases. From Type-M to Type-C, CaO content in core oxides increases, and Ca content in wrapping (Ca,Mn)S increases. The shapeof duplex (Ca,Mn)S inclusions can be controlled through controlling CaO content in core oxides. During solidification,CaO-Al2O3 oxides become as heterogeneous nucleation cores of MnS inclusions, duplex (Ca,Mn)S inclusions forming inthis way, and Ca in wrapping (Ca,Mn)S come from CaO in core oxides. The higher CaO content in core oxides, the higherCa content in wrapping (Ca,Mn)S. Under the condition with specific Ca/S ratio in steel, to obtain more duplex (Ca,Mn)Sinclusions, numbers of Type-C should be decreased, and numbers of Type-MC and Type-M should be increased. To achievethis goal, the key is to obtain larger numbers of CaO-Al2O3 oxides with lower CaO and smaller sizes in molten steel.