The effects of the actual diameters and diameter ratios of barrels and dies on the elastic swell and entrance pressure drop of natural rubber compounds in an extrusion capillary rheometer were investigated. Either the barrel diameter or the die diameter was altered so that different barrel-diameter/die-diameter (D-B/D-D) ratios were obtained, both the barrel and die diameters also being varied simultaneously. The extrudate swell and entrance pressure drop were dependent not only on D-B/D-D but also on the actual diameters used. For fixed D-B/D-D ratios, the change in the extrudate swell was linearly influenced by the entrance pressure drop at low actual barrel and die diameters (D-B/D-D = 20/4-30/7 mm/mm) but was associated with a change in the material viscosity at high barrel and die diameters (D-B/D-D = 35/7-40/8 mm/mm). When the die diameter was fixed, the relationship between the entrance pressure drop and the extrudate swell was linear tip to a certain value of the barrel diameter greater than 30 mm. Beyond this critical barrel Value, the relationship became nonlinear and associated with the shearing stress generated by the formation of semipluglike flow patterns and the residence time of the material. For a constant barrel diameter, the smaller the die diameter the greater the extrudate swell was because of the increases in the extensional deformation and wall shear rate coupled with a reduction in the material residence time.