Abstract: Penman Monteith equation was used to compute daily ET₀ for five stations: Dali, Yuanjiang, Baoshan, Kunming, and Jinghong, based on the history meteorological data range from 46 to 48 years, and monthly ET₀ series of the other 25 meteorological stations from 1961 to 2000. The most important influence factor of daily ET₀ is daily maximum temperature. The key influence factor of yearly ET₀ is sunshine, but is wind speed and maximum temperature in some stations. And the fog is not relative with ET₀. Then, Mann-Kendall method was used to test seasonal and yearly ET₀ series of five stations. Results indicate that yearly ET₀ series for 56.7% of weather stations increase significantly, which locates at Gengma, Simao, Menghai of Lancang River basin, and Weixi, Fugong of Hengduan Mountainous Area. Daily ET₀ in summer and autumn at Kunming station markedly decrease, while dialy ET₀ in winter and spring at Jinghong station increase, but daily ET₀ at Yuanjiang, Baoshan, and Dali station changes irregularly. The change trend of main meteorological factors is that precipitation and temperature increase, evaporation and sunshine decrease, moisture index increase too, which make ET₀ series of 80% stations decrease observably.