Monodispersed hexagonal Al2-xCrxO3 nanodisks are synthesized through a reactive doping of Cr6+ cations in a hydrogenated mesoporous AlO(OH)center dot alpha H2O powder followed by annealing at 1,200 degrees C in air. The reaction was carried out by a drop wise addition of an aqueous Cr6+ solution (0.5-1.0 M) to AlO(OH)center dot alpha H2O, at room temperature. Al2-xCrxO3 nanostructure formation was controlled by the nucleation and growth from the intermediate amorphous mesoporous Cr4+:Al2O3 composites in the temperature range 400-1,000 degrees C. The nanodisks of similar to 50 nm diameter and thickness of similar to 16 nm is observed in the sample with x of 0.2 and similar nanodisks with a low dimension is observed at a higher value of x of 1.6 (after 2 h of heating at 1,200 degrees C). The Cr3+ <-> Al3+ substitution, x <= 1.2, inhibits grain growth in small crystallites. The crystallites in x = 0.2 composition have 43 nm diameter while it is 15 nm in those with x = 1.2 composition.