Hydrocyclone is a device commonly used in mineral processing industry. Many studies attempt to understand size separation phenomenon but, none of them has taken an energy point of view. This paper presents the results of modeling underflow energy dissipation in hydrocyclone operation from a mechanical energy balance. The results related to newtonian suspensions show that underflow energy dissipation does not control the operation. That magnitude is small compared to interior energy dissipation but provides enough information to describe underflow conditions. The most important dimensionless parameters to this study about hydrocyclone energy modeling are exit diameters ratio, split ratio, normalized air core diameter and discharge angle.