Display omitted •111- and 100-oriented grains alternately form as growth proceeds in wedge-shaped cell.•Stripe patterns form between n-layer 100- and n + 1-layer 111-oriented grains.•The stripe pattern is formed by continuous change of particle configuration.•The interval of the strip pattern becomes long as the number of layers increase. The structural evolution of growing thin colloidal crystals in a confined space via the convective assembly technique has been investigated. The thin colloidal crystals were grown in a wedge-shaped cell, where the height of the cell increased with increased crystal growth. Triangle and square patterns, denoted as 111- and 100-oriented grains, respectively, were formed alternately as the height of the cell increased. The structural transformation was associated with an increase in the number of layers when the n-layer 100-oriented grains changed to n + 1-layer 111-oriented grains. Between the different grain structures, a stripe pattern was observed, which was a transitional region, where particle configuration gradually changed. The structural transformation occurred through the continuous change of particle configuration rather than through the abrupt formation of a grain boundary. The interval of the strip pattern lengthened as the number of layers increased, which is understood to be the structure with the highest packing density. The findings of the study give a better insight into convective assembly in a confined space, and also contribute to the greater structural control of colloidal crystals, useful for a number of applications.