The structure and movements of the air-core in a hydrocyclone play a key role in the performance and efficiency of separation. The technique of computational fluid dynamics has been used to evaluate the air-core characteristics and their influence on the internal flow field for the case of a 140 mmhydrocyclone. Through computational analysis for developed flow in a hydrocyclone, the shape, diameter, and steady-state distribution of the air-core were analyzed and presented. The steady-state distribution of the air-core could be divided into three distinctive parts. Two parts of the air core, on the cylindrical and spigot sections are very stable, and their shape and diameters do not vary over time. For the third part, on the conical section, the dense transient-state situation of the air-core has no clear rule. This results in the dense transient internal flow field, meanwhile, the internal flow fields of the other two parts are stable. Based on the calculation, the analysis of steady state of the air-core has been extended to operational conditions, including inlet velocities, viscosity, and atmospheric pressure. Those operation factors do not have any influence on the steady-state situation of the air-core in the developed flow of hydrocyclone. However, diameters of air core change with those factors. Any increase in the inlet velocity will increase the air-core diameter.