During the treatment process of drinking water, chlorine reacts with the organic matter present in water and forms various disinfection by-products (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs). The high content of THMs in the drinking water may be cancerogenic for humans and this has resulted in significant scientific and public concern. The aim of this paper was to determine the factors that have an impact on THM formation in the drinking water in the city of Tetova for the spring season 2011. Results of this research have shown that during high contact time, temperature, pH and chlorine dosage, more THMs were formed. Based on the examined factors, results show that contact time, pH and the chlorine dosage were crucial and had a significant role in THM formation. The presence of THMs was determined with UV-VIS spectrophotometry. This was the first study that researched the factors that impact the formation of THMs in the drinking water in the Republic of Macedonia. The study results match conclusions reached by previous research in the field.