Image analysis of soil section is often used by soil scientists in order to quantify the soil structure and macro-pore properties. During the computer image analysis the most difficult and error-burdened process is segmentation when pores and solid phase are detected. This paper presents a method of improving the image quality for computer analysis. Sodium hypochlorite was tested with respect to its bleaching effect for solid phase of soil. Different concentrations of NaClO and time of rinsing were tested. The results show that sodium hypochlorite increases the contrast between pores and solid phase and makes the process of image segmentation more precise. The optimum parameters of soil section bleaching are the concentration of 10 g l-1 available Cl and duration of 48 h.