In this study, to understand the starch granule size distribution in maize endosperm and the change of ADPG pyrophosphorylase activity during grain filling, we set up three planting densities (57 000 plants/hm2, 67 500 plants/hm2 and 78 000 plants/hm2) using the maize cultivars Longping 206 and Zhengdan 958 as materials. The results showed that the size of corn starch granules ranged from 0.4 to 30.0 μm, with increasing trend over the increase of planting density. The volume distribution of corn starch granules showed a bimodal curve, and a three-peak surface area distribution. With the increase of plant density, the volume and surface area percentage of medium starch granules decreased (3~18 μm) significantly, while the volume and surface area percentage of large starch granules (>18 μm) increased significantly. The distribution of starch granules showed a single peak curve, and the planting density had no significant effect on the distribution of starch granules of the two cultivars. The activity of ADPG pyrophosphorylase decreased in the filling stage of the two maize cultivars with increasing planting density.