Several 0.4C-Cr-Mo-Ni steels with different sulphur and calcium content levels have been studied to determine the effect of desulphurization and calcium treatments on the morphology of non-metallic inclusions in low-alloy steels. The volume fraction and size parameters of the inclusions were determined using optical microscopy. The inclusions were identified by electron probe microanalysis. The volume fraction and mean aspect ratio of a stringy type of inclusion, which consisted predominantly of MnS, was reduced significantly with a decrease in sulphur content through desulphurization. A cluster type of composite inclusion was also decreased with decreasing sulphur content. Calcium treatment of steel with a commercial sulphur level was not very effective for modification of the inclusions, producing two types of cluster composite inclusion. However, the calcium treatment of desulphurized steel modified dramatically the stringy type to a particle type (mean diameter : 1.3 mum) which consisted predominantly of CaS-CaO, while small amounts of Ti and Al were also detected.