The thermal performance of cooling towers is affected mainly by the velocity, temperature and humidity of the entering air, In this paper, the effects of these variables are experimentally investigated for both counter-flow and cross-flow cooling towers. The cooling performance is reduced by up to 50% as the relative humidity of the entering air is increased from 40% to 80%. The higher air velocity and lower coolant flow show better cooling performance. The coolant loss rates in the present experimental conditions are in the range of 0.4 to 1.7%