This paper proposes a new concept-Exciting Condition Index (ECI) for evaluating ride and durability tests of vehicls. As vehicle pass through a road stretch at a given speed, the spatial harmonics of the road surface will excite the vehicle. And the total effective accelerated power spectrum in time domain is defined as ECI.As ride and durability tests of vehicles suspension or its load bearing system carried out on natural road, then can not be compared with each other, but tests with same ECI value could do. So using ECI we can increase data base of ride and durability tests greatly. It can also be evaluated using ECI in service condition causing failure. A grading of ECI of durability and its recommended value are presented.Some examples of application are also briefly described in the paper.