Abstract: Abstract: In this paper, we applied an original image filter called BMO (bounded mean oscillation) and two other common image filters, the PM filter (Perona-Malik PDE model), and the median filter, to remove the noise and estimate the porosity and pore size distribution of digital images of a soil thin section obtained from three contrasting tillage systems (conventional tillage, rotary tillage, and zero tillage) and soil layers. The BMO filter uses a new derivative format to detect the boundary characteristics of images, and eliminates the influence of image noise using the mathematical method of integral average. The results showed that the BMO filter performed better than the other two filters in detecting image edges and detail characteristics. Compared with the other two techniques, the BMO filter gave a significantly higher porosity for the soil pore with aperture < 5 μm, and the soil porosity obtained from the BMO filter was the closest to the experimental data. The PM filter produced higher porosities for the pores with aperture 5 to 50 μm and > 50 μm than the other two filters, but no significant differences were observed among them. It followed that the PM filter was a little better than the other two filters in analyzing pores with aperture >5 μm. There were no remarkable differences between the soil porosities with aperture > 50 μm obtained from the three image filters and experimental data. However, for soil pores with aperture < 50 μm, porosities obtained from the image filters and experimental data were significantly different. The main reason for that was that the two-dimensional soil slice could not completely represent a three-dimensional structure of undisturbed soil samples. Image resolution may also impact on identifying small soil pores. The advantage of a BMO filter on identifying small soil pores to some extent compensated the loss of small soil pores caused by a preparation of soil section and image resolution. Our results also indicated that the conventional and rotary tillage systems improved soil porosity and the ratio of transmission pores (pores with aperture > 50 μm) in a surface soil layer. Zero tillage decreased soil porosity due to the reduction of mechanical disturbance on a surface soil layer, and showed beneficial effects on increasing the ratio of storage pores (with apertures of 5 to 50 μm).