This study is concerned with a kind of oil-free mechanical vacuum pump with spiral grooved rotors. To improve the performance of the pumps at pressures above 1000Pa, sleeves which have triangular or rectangular grooves were tested instead of the smooth ones ordinarily used. The pressure difference between suction and discharge sides was measured for various pairs of rotors and sleeves. The measured values were normalized and plotted against the Reynolds number defined as Re=4ρUm/μ(U : peripheral speed, m : hydraulic mean groove depth, ρ : density, μ : viscosity). Grooves on sleeves serve as paths for leakage in the laminar regime (Re<1000), but they activate the generation and conveyance of vortices when the flow becomes turbulent, and thus improve the performance. The improvement becomes marked as the groove pitch on the sleeve decreases and as the helix angle of the groove on the sleeve increases.