Pesticide is any toxic chemical substance used to kill animals, fungi, or plants that cause economic damage to crop or plants or are hazardous to the health of domestic animals or humans. Increasing amounts of pesticides that are introduced into the environment cause an increased concentration of harmful organic and inorganic compounds in water, air, soil and industrial waste streams, which is why photocatalytic processes are increasingly used for the decomposition of harmful compounds. The aim of this study was to analyze and compare the efficiency of photocatalytic degradation of thiacloprid under different operating conditions and to examine the possibilities of improving the degradation efficiency. Photocatalytic degradation of thiacloprid was performed in a flate-plate photoreactor using an immobilized TiO2 as a photocatalyst. The influence of the pH value of the initial reaction mixture on the efficiency of photocatalytic degradation of thiacloprid was investigated, and for the purpose of comparison, the reaction of photolysis (degradation without the presence of photocatalyst) was performed. It was found that the degradation of thiacloprid is most effective at an initial pH of the reaction mixture of 4,5-5,5 and that photolysis (without the presence of photocatalyst) results in minimal degradation of thiacloprid. Also, it was found that pre-ultrasonic treatment of the reaction mixture increases the degradation efficiency.