UV irradiation technology has the advantages of ease operation, affordable, and clean production. This paperfocused on the wettability improvement of cotton yarns by UV irradiation, and proposed UV irradiation pretreatment methodto improve the sizing performance of cotton yarns. The surface elemental composition and molecular structure before andafter UV irradiation were characterized. The effects of UV irradiation pretreatment intensity and time on apparent properties,weight, wettability and mechanical properties of raw cotton yarns were analyzed. The impact of UV irradiation pretreatmentprocess on sized yarns performance including mechanical properties, hairiness, yarn evenness and size pick-up were studied. Then, the mechanical properties of the sized yarns after alkali desizing were investigated. The results showed that UVirradiation pretreatment can introduce oxygen-containing polar groups on the cotton yarns. Meanwhile, it had a certaindegradation effect on the surface of the cotton yarns, which resulted in wax removed, roughness improvement and exposureof cellulose. The above reasons jointly caused the wettability improvement of raw cotton yarns after UV irradiation. Compared with the unpretreated sized ones, the breaking strength, breaking elongation and abrasion resistance of the UVirradiation pretreated sized yarns was greatly enhanced by 20.13 %, 20.54 % and 103.49 % respectively. The hairiness wasreduced by 88.23 %, and the yarn evenness was improved by 6.74 %. It presented the optimized sizing performance when theyarns were pretreated at UV irradiation intensity of 142.5 mW/cm2 for 120 min. After desizing, the decline amplitude waslittle between unpretreated and UV irradiation pretreated yarns.