Abstarct: In Power Plant Industries, frequently hydraulic coupling are used to transmission of power from electromotors to the high power pumps and ventilators (fans) due to the lack of physical confrontations in compared with mechanical couplings. However, because of the oil trapped in the coupling region of a continuous function and thermogenesis, the physical and chemical properties of the oil has lost and turns in to the mud (an early disability of coupling) that problem can be greatly delayed by correct analysis of thermal behavior and proper cooling. Thus, in this paper, thermal and hydrodynamic behavior of mixtures of oil and air in a power plant hydraulic coupling simulated and assessed by FLUENT software. The aim of this study is to identify the locations of the secondary flows and vortex which has the greatest role in thermogenesis .and also, determine the areas (fluid and body coupling) with high temperature. In this simulation, the inside factor fluid flow of coupling is examined in three-dimensional mode, biphasic systems and turbulence. The velocity and temperature distribution inside the fluid and the temperature distribution on the body of coupling are calculated and analyzed. The simulated results indicate that the maximum temperature is occurred at the outer diameter of turbine coupling (where the eddy and secondary flows occur). At the end, the effect of increase of distance between the pump and turbine wheel and also the effect of body thermal fins displacement on the performance of coupling and the reduction of thermal and hydraulic dissipation has been studied.