In the current paper the dimensional analysis of thermo-fluid-dynamic mechanisms taking place during high pressure treatment of bio-substances is carried out. Both forced and free convection case is described. Additionally, the phase change in the pressurized medium is considered. The significance of several terms in the conservation equation of momentum and energy is estimated. Especially in systems with low velocity of fluid some terms in the governing equations can be neglected, e.g. irreversible transformation of kinetic in thermal energy. The dimensionless numbers/groups that describe the fluid flow and phase transition at high hydrostatic pressure are determined. (C) 2007 Elsevier Ltd. All rights reserved.