In this research, a complete open hydraulic drive mixer system has been designed, instrumented and commissioned, and an extensive programme of experimental tests has been undertaken to 1)- investigate the effectiveness of a cooling unit as an integral part of the open hydraulic system and 2)- validate the mathematical model. The results have shown that the working temperature could be reduced by 40 % by using the integral cooling/reservoir unit and the temperature is always kept below the recommended operation temperature. A mathematical model for temperature distribution under unsteady state conditions in an open hydraulic systems has been developed to predict pipe wall and fluid temperatures in the system. The thermodynamics processes and heat transfer by convection, conduction and radiation have been taken into account. The developed temperature transient equations are solved by using numerical integration technique which are used widely in computer programming. A software package has been developed to be used in hydraulic system design. The main advantage of this package is the user friendliness. The simulation results shows a significant difference between the temperatures of the fluid and the pipe wall in the hydraulic systems and demonstrated that this mathematical model is more accurate than those reported elsewhere. The main results of this investigation is that the hydraulic reservoir has been reduced in the size to about 15 percent of the conventional reservoir in the open hydraulic systems. Furthermore, the experimental results have shown a close agreement with the theoretical results.