A mathematical model of the performance of a cooling tower is presented. The model consists of two interdependent boundary-value problems, a total of 9 ODE, and the algorithm of self-consistent solution, The first boundary-value problem describes evaporative cooling of water drops in the spray zone of a cooling tower; the second boundary-value problem describes film cooling in the pack. Simulation of the boundary-value problems has been made. The comparison between experimental data and calculated results showed that the model correctly describes the basic regularities of the cooling tower performance. In the effective droplet-radius approximation the difference in the thermal efficiency between calculated and experimental results does not exceed 3%. The limits of applicability of the developed mathematical model and its possibilities are discussed.