Abstract:In this paper, a new quantitative model of natural selection pressure was derived. Then, 14 natural populations of Populus cathayana and 6 provenances of Pinus sylvestris var mongolica were sampled, and their α, β- esterase isoenzymes were & analysed. In the meantime, by which some data from three experiment published in silvae, genetica were analysed and compared it with other methods. It shows that this theory can solve some problems which cannot been solved by other methods. Besides these, it has other advantages below: 1. very convenient statistical analysis, 2. wide application, 3. easy cooperation with other subjects, 4. great potentialities to be used in many primary biological studies, especially in population genetics.