DIW (Dynamometer Inertia Weight) of 'chassis dynamometers for automobile emissions testing' is a total inertia weight of all rotating components in the chassis dynamometer. Total Inertia weight is a inertia device simulates translational and rotational kinetic energy of a traveling vehicle which is equivalent to the mass of the car. DIW is an important technical indicator, whether it is accurate or not will affect the calibration of all technical characteristics of 'chassis dynamometers for automobile emissions testing'. In this paper,first of all, we introduce a new method to measure the Dynamometer Inertia Weight, that is, coast-down testing with twice constant-force loading method, and we derivate a formula to calculate the Dynamometer Inertia Weight from kinetic energy conservation law. Secondly, we have done a lot of coast-down testings with twice constant-force load method. After analyzing the data, we found out the factors that affect the testing processes and the accuracy of testing results. Finally, after comparing twice constant-force loading method with the take-down flywheel method and twice constant-power loading method, we know that this coast-down testing with twice constant-force loading method is better than other methods in stability and repeatability and testing data will be more accurate. It's a accurate and convenient way to measure the DIW of 'chassis dynamometers for automobile emissions testing'.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.