Influence of additive material to surface roughness in turning of sintered steels is not well known. In this study, effect of manganese sulfide (MnS), silica and alumina powder was investigated on a viewpoint of material characteristics near turned surface. The surface roughness of the sintered steel added with MnS was smaller than the others. MnS was extended and dispersed finely in the deformed layer near the turned surface, while silica and alumina were localized but not extended. Nano indentation test showed that the deformed layer of the sintered steel added with MnS had lower hardness in comparison with others. We considered that a tear mark of the cutting chip separated from deformed layer was lowered by MnS and the trace of torn chips, namely the surface roughness became relatively small. However, the deformed layer of the sintered steel added with silica or alumina was close to that of the additive-free sintered steel and the surface roughness became similar to these cases.