Roots rotors dramatically determine the performance of Roots vacuum pumps. In order to develop new rotors with higher performance, a pair of novel asymmetrical rotors or eccentric involute rotors were proposed based on the conventional symmetrical involute rotors. Profiles of one rotor consist of four eccentric involutes and four circular arcs; profiles of the other rotor consist of four conjugate curves of eccentric involutes and four circular arcs. A model of the proposed asymmetrical rotors was established, and effects of geometric parameters on the area-utilizing ratio of rotors were analyzed. The study results show that the proposed asymmetrical rotors have more number of independent geometric parameters than the conventional rotors of Roots vacuum pumps, so that the asymmetrical rotors are superior to the conventional rotors in terms of design flexibility. The contents of this study can be applied in the design of Roots vacuum pumps.