The cluster size of water molecules in poly(vinyl alcohol) (PVA) film was determined using the cohesive energy equation and the heat of evaporation of water. On the heating process of PVA film, the mass of evaporated water was measured by thermogravimetric analysis (TGA) and the heat of evaporation was measured by differential scanning calorimetry(DSC). The evaporation energy and the cluster size are closely related each other, and the size was calculated using the equation of cohesive energy which contains surface energy. As a result, the size of water cluster in 0% relative humidity (RH) film was about 13.2 molecules, 40% RH film was 6.6 molecules and 80% RH film was 3.5 molecules, respectively. The cluster size in 0% RH and 40% RH films are likely to contain errors caused by the overestimation of evaporation energy. In the film of 80% RH, the clusters of 3.5 molecules were absorbed at a ratio of one cluster per 5.3 repeating units of PVA chain. The cluster size was calculated at various temperatures in the order of evaporation. The average cluster size in 80% RH film was about 1-2 molecules on the film surface and it was about 3.5 molecules in the inside of film.