To grasp the temporal and spatial distribution characteristics of the pan evaporation amount in the Chao Phraya River basin, in the Kingdom of Thailand, the writers performed an analysis over a period of 19 years, from 1982 to 2000, where data related to the target area were available. Geographic, seasonal, and annual changes were studied. As a result of performing trend tests on annual changes, negative trends were found in 24 out of 27 observation stations. Furthermore, nine of those locations had a significant negative trend. As a result of performing tests of concurrent rainfall over the same period, although there were positive trends in 8 locations and negative trends in 19 locations, no significant trend - either positive or negative - was obtained. This result does not match a finding that precipitation increase decreases pan evaporation in the United States, a finding that can be derived from the complementary relationship. Journal of Hydrologic Engineering © ASCE.