Specific grades of high-entropy alloys (HEAs) can provide opportunities for optimizing properties toward high-temperature applications. In this work, the Co-based HEA with a chemical composition of Co47.5Cr30Fe7.5Mn7.5Ni7.5 (at%) was chosen. The refractory metallic elements hafnium (Hf) and molybdenum (Mo) were added in small amounts (1.5at%) because of their well-known positive effects on high-temperature properties. Inclusion characteristics were comprehensively explored by using a two-dimensional cross-sectional method and extracted by using a three-dimensional electrolytic extraction method. The results revealed that the addition of Hf can reduce Al2O3 inclusions and lead to the formation of more stable Hf-rich inclusions as the main phase. Mo addition cannot influence the inclusion type but could influence the inclusion characteristics by affecting the physical parameters of the HEA melt. The calculated coagulation coefficient and collision rate of Al2O3 inclusions were higher than those of HfO2 inclusions, but the inclusion amount played a larger role in the agglomeration behavior of HfO2 and Al2O3 inclusions. The impurity level and active elements in HEAs were the crucial factors affecting inclusion formation.