The densification rate of pores located at grain corners is obtained by constructing and analyzing a simple model with a transport mechanism of grain-boundary diffusion. The densification rate obtained for a microstructure of tetrakaidecahedral grains decreases with densification and is nearly consistent with the Wilkinson model at low porosities. The effect of grain-size distribution on the densification rate is also estimated under the two limiting conditions of constant stress and constant strain rate. At constant strain rate, the densification rate with grain-size distribution is lower than the value for the average grain size, whereas it is identical to the value at constant stress.