In hydraulic systems, the property of hydraulic oil transferred power will change accordingly when the temperature and the pressure of the hydraulic oil and the air bubbles dissolved or mixed into it change. Therefore, the performance of the hydraulic systems will change. In order to design the hydraulic system which is responsive quickly, running steady and efficient the property of the hydraulic oil is studied in this paper. The mathematical model of the hydraulic oil will be constructed. This model will provide base for study of simulation of the hydraulic system.