A sintering model, taking into account the effect of particle-size distribution and the effect of grain growth, has been derived. The model predicts a dependence of densification on the width of the particle-size distribution. This dependence is strongly affected by the occurrence of grain growth. Prior to the occurrence of grain growth, the model predicts that the densification rate increases and then decreases as the particle-size distribution width of the original powder increases. After grain growth occurs, the densification rate decreases as the particle-size distribution width of the starting powder increases.