Abstract: A new method was proposed to determine the moment of inertia of the sprung mass of a car around its mass center. An on-the-road brakeage test should be done first. Analyzing the characteristics of the frequency spectrum of those vertical vibration acceleration signals which were acquired from the test, the inherent angular frequency was obtained. A vibration model was thus developed for car body to calculate the moment of inertia. A SY622B minibus was taken to exemplify the model and the calculated value with measured value of the moment of inertia. The error is shown only 1 % that will meet completely the precision requirement in engineering. The method is accurate and applicable to the measurements of moment of inertia of various types of car.