Abstract: Quantification of root distribution contributes an improved understanding of chemical fluxes in the vadose zone to both ecological and hydrological studies.Based on the data observed at Ejina Banner,Inner Mongolia of northern China,from May to July of 2006,root distribution of Populus euphratica forests was analyzed.Root system was divided into coarse root system(d>0.2 cm) and fine root system(d≤0.2 cm).The content and method of research were different between two root systems: 1) for coarse root system,using fractal theory,the authors researched the relations between root distribution of P.euphratica and soil moisture.And a new definition,expectation of soil moisture content,was introduced.When the mean value of expectation of soil moisture content was 25.03 cm·g/g,the fractal dimension of coarse roots was 2.000 7,and when it was(55.95 cm·g/g),then the fractal dimension was 1.627 5,ie the larger the expectation of soil moisture content was,the smaller the fractal dimension of coarse root distribution was,vice versa.2) For fine root system,one-dimension(including the vertical and horizontal radial direction) and two-dimension function of root length density(RLD) were simulated.All the functions of RLD were simulated by exponent function,and R² was(0.89),0.68,0.73 and 0.69, respectively.It shows that the RLD of fine root system of P.euphratica is accorded with exponent distribution on the whole.