Abstract: This paper presents a method to determine a fractal dimension associated with microstructure of clayey soil by image-processing techniques and Geographical Information System (GIS) software. And the method of determining threshold in image-processing is discussed. Through analysis of SEM of clayey soil of Yun County of China, a conclusion is drawn that the microstructure of clayey soil is fractal, and the relationship between the fractal dimension and the microstructure of clay soil is invertigated.