A mathematical model for solid particle degradation by an aerobic biofilm of Bacillus coagulans is developed. A moving biofilm is assumed to be present on the surface of the solid particle. Oxygen and glucose are assumed to be growth limiting. The depleting glucose concentration in the solid particle is tracked as a function of time and it is found that the time taken for degradation of the particle is a function of particle size. Comparison with the experimental results found in literature on the particle size reduction by the action of Bacillus coagulans (Nandakumar et al., 1996) indicates that the model is able to predict the general trends of experimental data well. It is suggested that the diffusion of the enzyme glucoamylase plays a crucial role and is a more dominant determining factor than the variation of the composition of particles with size as suggested previously (Nandakumar et al., 1996) for the experimental observation that larger particles took more time to degrade than smaller particles. It is hoped that the results obtained will lead to a better understanding of the mechanism of particle degradation by aerobic biofilms and help in better design of biofilm reactors.