Internal mixers are commonly used for rubber compounding in industry. However, the relations among the designs of equipments, mixing conditions and the properties of the resulting compound are little known. In this paper we tried to reveal some manners of the relations concentrating, particularly, on rotor designs. A specially designed internal mixer (Banbury type) was developed. A number of double winged rotors of various shapes and replaceable mixing chambers were prepared. In developing the equipment, eight parameters were taken into consideration concerning the shape of rotors, that is, lengths, twist angles and overlap ratio of the wings, width and clearance at the rotor tip, angles of flow-in and flow-out at the gap area and total bulkiness of the rotor. Measured properties concerning mixing are: unit work, Mooney viscosity of dumped compound, the amount of bound rubber, weight average molecular weight of the extracted portion of the compound, and peaks in a torque curve, i.e., values of first peak torque and second peak time. In order to estimate the degree of influences of the design parameters mentioned above and mixing conditions, each measured properties were expressed as functions of these parameters by means of multiple regression analysis.