Abstract: A novel rotor with higher shape-factor was proposed to decrease the gross weight and to increase the volume utilization coefficient of Roots vacuum pump with flat-convex rotor profile.The volume utilization coefficient and shape-factor of the straight-valley rotor,straight-top rotor and novel rotor with higher-shape factor were analytically derived and calculated,respectively.For example,the shape-factor(the volume utilization coefficient) of the new rotor,at an angle of 2° and with 2,3 and 4 lobes,were 1.413,1.396 and 1.370(0.4865,0.4732 and 0.4549 and with the increase of 14.39%,17.92% and 22.22%),respectively.The advantages of the proposed rotor with higher shape-factor include greater volume utilization coefficient,lighter weight,smaller radial/conjugate leakages,lower center of gravity,higher dynamic stability,less noises and weaker vibration.We suggest that the calculated results be of basic and technological interest in design optimization of lightweight Roots vacuum pumps.