The purpose of the bachelor’s thesis was to determine whether the avocado pit extract can be used as a natural reducing agent for in-situ synthesis of zinc oxide nanoparticles (ZnO) on cotton fabric and whether good protection against ultraviolet (UV) radium can be achieved. For this purpose, the molar concentration of the precursor (zinc chloride (ZnCl2)), the order of treatment of the cotton fabric with the precursor and reducing agent, and the temperature of the synthesis were studied. On the UV/VIS spectrophotometer, the sample was determined by the UV protection factor (UZF) and the color of the samples (CIELAB) on the reflection spectrophotometer. The morphology of fibers or synthesized nanoparticles on fibers was studied by linear electron microscopy (SEM). The samples were measured in color and were determined by UZF after a five-time household wash. The results of the study showed that the in-situ synthesis temperature was important and samples had higher (UZF) values if they were treated at the room temperature than at 60 °C. The order of processing of the fabric with the precursor and reducing agent and the molar concentration of the precursor were also influenced by the formation of nanoparticles and UZF values. Higher values of UZF had samples where 1 M ZnCl2 solution was used than 0.5 M. In the case where the in-situ synthesis was carried out in such a way that the sample was first treated in the precursor and then in the reducing agent, the sample showed the highest protection against UV radiation (good UV protection). The sample was not among the darkest, so the color was not the one that affected the UZF but the numerous nanoparticles of Zn formed on the surface of the cotton fibers, which was also evident from the SEM analysis. The order of processing of the fabric where the fabric was, first, treated in the reducing agent and then in the precursor gave the worst results of the UZF. From the SEM analysis, it can be seen that ZnO nanoparticles were formed on the fibers, but apparently not in sufficient quantity for the treated fabric to provide protection against UV radiation. After washing, some patterns increase UZF, which is surprising. It is anticipated that the nanoparticles migrated to the surface of the fibers by washing and, therefore, increased the UZF.