Denitrification may appear in soil anaerobic regions. In order to estimate the anaerobic fraction of a soil, a finite element method was used to simulate 02 diffusion and consumption in topsoil layers. The soil is considered as a set of aggregates of various shapes and dimensions. Their external transport areas are reduced by contacts between the aggregates and by areas covered with water in the intercrumb pore space. In some simulations, the role of water from the intercrumb pore space in partially occluding the aggregate surface area has not been taken into account. In this case, anaerobiosis only becomes significant near saturation and is due to the reduction of the intercrumb O2 diffusion. When water of the intercrumb pore space is considered, anaerobiosis appears at lower water contents and is due primarily to the reduction of the aggregate external transport area. In this case, O2 concentration in the intercrumb pore space is high at all depths, except near soil saturation. The results of this study were compared with some published relationships between the water saturation level and denitrification, obtained from experimental data. There is a good agreement between our calculations and these functions. In addition, our simulations show that the relationship between the soil anaerobic fraction and the water content is highly dependent on soil structure, soil temperature, and microbial activity. Thus, multiplicative functions are inadequate to describe the effects of water and temperature on soil anaerobiosis or denitrification.