This paper deals with modeling of dynamic behaviors of friction in the sliding regime. Using a hydraulic cylinder, friction characteristics are experimentally investigated under various conditions of velocity variation. The friction force of the hydraulic cylinder is measured based on the equation of motion using measured values of the pressures in the cylinder chambers and the acceleration of the hydraulic piston. Comparison between measured unsteady-state friction characteristics and those predicted by the LuGre model shows that the LuGre model cannot simulate the real friction characteristics of hydraulic actuators. In this paper, the LuGre model is modified taking the dynamics of lubricant film formation into consideration. The lubricant film dynamics is approximated by a first-order lag element and its time constant is varied among the acceleration, deceleration and dwell periods. Experiments make clear the dynamic behaviors of the friction of the hydraulic cylinder. It is shown that the modified LuGre model can simulate the real dynamic behaviors of the friction of the hydraulic actuator with a relatively good accuracy.