Air swirl speeds were measured in a motoring and firing run of a direct injection diesel engine, and the results were compared with those in steady flow tests. Experimental results show that, during the combustion period, the swirl speed in the firing condition is almost the same as that in motoring. There is a linear correlation between the swirl speed predicted from the results of the steady flow tests and that measured at top dead center in motoring. Therefore, it is recognized that the steady flow test is available to evaluate a swirl producing ability of inlet system of engine. The reduction in swirl speed during the compression period was estimated by considering the friction force acting at the air-solid interfaces.