This paper investigated vertical wicking in twisted yarns. A mathematical model was developed based on a capillary penetration mechanism. By using a macroscopic force balance approach, the wicking time was derived as a function of the capillary rise of the liquid. Packing of fibers in the yarn was assumed to be uniform. In order to validate our model, a series of experiments was conducted on polyester yarns. The results showed a good agreement between the experimental data and the theoretical predictions. The influence of twist level of the yarn on the capillary flow was also investigated.