Mixing spectrum is proposed as a new index to express the mixing performance from a new viewpoint. The mixing spectrum is a terminology by the authors and the concept was built by making use of whole mixing capacity index. That is, the mixing spectrum is interpreted as the contribution from each scale of moving distance of fluid to the whole mixing capacity of an impeller. Specifically, the mixing spectrum can be obtained based on the value of whole mixing capacity index that is calculated by changing the value of the smallest scale of moving distance of fluid little by little. From the results of a new index, it can be said that the whole mixing capacity increases as the impeller rotation speed increases regardless of impeller style, there is no noticeable influence of the impeller rotation speed to the probability density distribution of moving distance of fluid and the mixing spectrum index distribution in the position of the relative distance specific to each impeller and the value of mixing spectrum index at the peak increases according to the impeller rotation speed because of the increase of the value of the whole mixing capacity.