The size distribution of quartz grains in soil is important in revealing the source of parent material and in simulating soil structure. In this study digital image processing method was used to extract the information of quartz grains based on their optical characteristics, and fractal dimension was used to characterize the number-size distribution of quartz grains. Soil samples under no tillage and mouldboard tillage were collected and then soil thin sections were made. Quartz grains in soil thin sections were segmented from cross plain light photographs by image analysis, the number, size and shape of quartz grains were measured with Sigma Scan Pro 5 software. The fractal dimension was calculated and its relations with the total number, total area, and average grain size were analyzed. The number-size distribution of quartz grains showed fractal characteristics with fractal dimensions between 1.60 and 2.18. The fractal dimension was negatively correlated with the total area and the average grain size. No significant difference was found between no tillage and mouldboard tillage treatments.