A mixing and kneading machine including a pair of parallel rotors each having a long vane and a short vane, both of which extend spirally about the center line of the rotors. A length ratio of the short vanes to the long vanes is selected within a range of 0.48 to 0.1. Also described is a mixing and kneading device which has a pair of parallel rotors rotating in opposite directions within a mixing chamber defined by a casing and an end frame. Each of the rotors comprises a long vane and a short vane, both of which extend spirally about the center line of the rotor, the spiral direction thereof being selected such that material to be mixed flows from the edges of the rotors into the central portion thereof. The ratio of the length of the short vane to that of the long vane is determined within the range of 0.48 to 0.1 and a ratio of axial thrust exerted on the material by the short vane to that by the long vane is determined between 0.7 and approximately 0.