The cooling water from a local oil refinery was sampled, analyzed and experimentally treated with UV light irradiation at a wavelength of ~254nm. Total Organic Carbon (TOC), Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and bacteria count (BC) were analyzed for the water samples over years to monitor the water quality. The experimental results showed that: 1) The cooling water quality (e.g. organic matters) varies significantly over the years’ operation, probably due to the change in weather (mainly the temperature); 2) The UV treatment was highly effective for the disinfection of cooling water. Within a 24 hour treatment period, it was observed that the bacteria count decreased from 3890 cfu/ml to practically undetectable, which is consistent with the UV/Vis analysis that the absorbance at 254 nm was decreased significantly over the time. The degradation of chemicals by UV was also observed in the experiment.