Abstract: The morphology, size, composition, and number of inclusions in pipeline steel were characterized by scanning electron microscopy and energy dispersive spectroscopy. The characteristics of the inclusions with different Ti/Mg ratios as well as the influence of sulfur content and the number of deoxidation products on the precipitation behavior of MnS were investigated, and the corresponding thermodynamic calculations were carried out. It is found that the core of the inclusions is mainly composed of MgO-Al₂O₃-Ti₂O₃, MgO-Ti₂O₃ or MgO, and then MnS wraps or locally precipitates on them. The inclusions have the average size smaller than 1.3 txm and the number of 300 to 450 mm^-2; moreover, their morphology is diverse such as round, polygonal and square. When the Ti/Mg atomic ratio is 0.05 to 0.2, the inclusions are round and small. With the decrease of sulfur content, MnS less tends to precipitate during solidification, its ways of precipitation on the inclusion surface will change from wrapped to local precipitated. A great number of oxides is conducive to the heterogeneous nucleation of more finely MnS on the surface of complex oxides. The Ti-Mg complex deoxidation products are small and dispersed, can become the heterogeneous nucleation core of MnS, and may reduce the harm of MnS and oxides at the same time.