Abstract: In order to quantitatively characterize and study the void structure of cellulose acetate (CA) filter rods, a capillary model was adopted and improved. The void inside a CA filter rod was regarded as a bundle of uniform empty capillary tubes. The radii and number of capillaries in the filter rod model were calculated via measuring five parameters, including rod circumference, mass, length, plug wrapper thickness and the amount of moisture absorbed by the capillary. The void structure of different CA rod samples was analyzed. The results showed that: 1) The method featured good repeatability with RSDs of tested capillary radius and numbers less than 0.5%. 2) There were about 1 600-2 200 capillaries with the radii of 78-90 μm inside a conventional filter rod(circumference 24.2 mm), and there were about 180-310 capillaries with radii of 140-190 μm inside a slim filter rod(circumference 17.0 mm). 3) The greater the pressure drop of a filter rod during processing, the higher the number of capillaries inside the rod and the smaller the capillary radius would be. 4) The numbers and radii of the capillaries inside the filter rods of the same specification (with the same length, circumference and pressure drop) processed with different tows differed significantly.