In the present study, for the first time, a full-scale experimental study of an ozonation contactor performance in a Municipal Surface Water Treatment Plant (MSWTP) in Hamedan, Iran was carried out over the course of one year. Effects of seasonal water characteristics and operating parameters were investigated on the removal of organic contents, microorganism destruction and formation of ozonation by-products, especially aldehydes and ketones. It was observed that the highest Total Organic Carbon (TOC) removal occurs during the ozonation step. The results indicated that temperature, pH value, ozone/TOC dosage, and contact time are the most important parameters that affect TOC removal and formation of ozonation by-products. It was observed that turbidity has a slight effect on pH value and TOC removal in the ozonation contactor. The ozonation by-products study showed that ozonation causes an increase of approximately 562% in the total aldehydes and ketones concentration in water that can lead to human health risks. Formaldehyde, acetaldehyde and acetone are the major by-products formed during ozonation (about 60%). Although the concentration of disinfection by-products in MSWTP does not exceed the accepted values in relevant standards, but the reactions of aldehydes with chlorine in post-chlorination step may produce halogenated by-products. These results indicated that actual concentration-contact time (CT) values and ozone dosages in the contactor are much lower than required value for the inactivation of pathogenic microorganisms. Finally, based on this study, some suggestions were made to improve the performance efficiency of the ozonation contactor and control disinfection by-products formation.