Probability that the grains of cohesionless soil in an investigated range of grain diameters can be eroded from a volume of soil by the groundwater flow force is estimated using a two-person zero-sum game theory. The estimation of probability of erosion of soil grains is based on the two dimensional model of a soil structure, which is derived from the soil grain-size distribution and measured soil porosity, as well as on the assumption that soil grains are circular in their cross sections. A comparison of the results obtained by proposed method and by experiments shows good agreement.