Abstract: The one of the task of plant community research is environmental interpretation for communities. The quantitative analysis of vegetation-climatic relationships is the most important link of it. Potential evapotranspiration (PE), as the integrated parameter for the two most significant ecological factors--heat and moisture and the quantitative index which connects plants and their environment, has attracted serious attention from ecologists, geographers, and climatologists. This paper deals with several significant and successful methods for calculating PE and vegetation-climatic classifications, such as, Penman, Thornthwaite, Holdridge, and Kira, s equations or arithmetical systems. The appropriate computer programs (PEP) are attached for the convenience of the users.