Uniformity of mixed rubber in an internal mixer equipped with three long-type rotor sets was investigated. After mixing butadiene rubber with ZnO powder for a certain period of time, a small amount of the mixed rubber was taken out from two points in the mixer: the center point of the reservoir and the side point of the sickle-shaped zone on the rotors. The concentration of ZnO in the sampled rubber was measured by the atomic absorption analyzer to know the dispersion of ZnO into the rubber. The twist direction and helix angle of rotors, and the position of ZnO powder incorporation, affected the uniformity of mixed rubber. (C) 2004 Wiley Periodicals, Inc.