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Leakage characteristics of the screw gas seal having a rectangular screw groove on a shaft were investigated by varying the peripheral velocity up to 250 m/s and by using room temperature air. The various effects of helix angle, groove width ratio, aspect ratio, number of threads, etc, were examined both in stationary and rotating states. As a result, it was found that the seal performance was optimized by using a dimensionless parameter, the ratio of the cross-sectional areas of screw groove to screw rotor. Thus, the seal performance was optimized when this parameter was nearly equal to 1 × 10-3 or when the cross-sectional area of screw groove became nearly 0.1% of the cross-sectional area of screw rotor. The effects of aspect ratio, helix angle, groove width ratio and number of threads on the seal performance appear to be relatively unimportant.