The aim of the paper was the modelization of the influence on the Ultraviolet Protection Factor (UPF) of the more representative properties of the wearing conditions of the garments, the tension produced in contact with the body and the wetness. The study was carried out on undyed woven fabrics manufactured with three different cellulosic fibers (Cotton, Modal and Modal Sun) and with three different structures. The transmission of Ultraviolet Radiation (UVR) in the whole ultraviolet range, as well as in the UVA and UVB regions, was measured and the UPF was determined. A statistical model for each fiber type was formulated, which allowed the prediction of the UPF according to the UPF of the original fabric (unstretched and dry), the tension and the wetness.