The success of hydromounts in vehicle vibration isolation during the past decade and a half has stimulated automotive engineers to apply the similar mechanism to bushing design. A typical conventional bushing is comprised of a rubber section enclosed by an inner and an outer metal, while a bushing filled with internal fluid is called a hydrobushing. Based on the fact that the fluid inside the hydrobushing generates the majority of the damping, the mathematical model of the hydrobushing used in this paper neglects the rubber damping effect to the system. The equations of fluid motion and conservation and system equilibrium equation determine its resonant frequency. The conclusion is that the resonant frequency is proportional to the ratio of the cross sectional area of the inertia track to the equivalent piston area and the square root of volumetric stiffness; and it is inversely proportional to the square root of the mass of the fluid in the inertia track. The predicted resonant frequency correlates well with the tested results.