The cooling system is an essential part of the production process within the plant. In the cooling system using cooling tower, efficiency can be improved by increasing the effectiveness of cooling through nozzle modification. This study aims to determine the effect of diameter and number of nozzle holes on cooling effectiveness. The data were taken from the experiments using prototype of counterflow cooling tower with dimension of high x cross-sectional area are 120 cm x 40 cm x 40 cm. Diameter varied by 1; 1.5; and 2 mm, while the number of nozzle holes 4; 8; 16. The results show that the highest cooling effectiveness was achieved at nozzle diameter 1.5 mm with the number of holes 8 that is 38.77%.