The slip velocity of molten rigid PVC was investigated. Using Mooney’s method, slip velocity was obtained from flow curves which were measured by slit dies with different thicknesses. When surface roughness of dies was 0.1 μm, the slip velocity logarithmically increased with shear stress. When an extrusion temperature was 180 °C, the slip flow was dominant and the ratio of slip velocity to mean velocity was about 90%. Its ratio decreased with increasing extrusion temperature. The slip velocity rapidly decreased when the surface roughness was increased from 2 to 10 μm. Over 10 μm of the roughness the apparent slip velocity became zero. Therefore the thickness of the slip layer was supposed to be within several microns.