The structures and hydrogen storage capacities of (AlN)n (n = 3-5) clusters have been systematically investigated by using density functional theoretical calculations. At ωB97xD/6-311 + G(d, p) level, the planar structures of (AlN)n (n = 3-5) can adsorb 6-10 H2 molecules with average adsorption energies in the range 0.16 to 0.11 eV/H2, which meet the adsorption energy criteria of reversible hydrogen storage. The gravimetric density of H2 adsorbed on (AlN)n clusters can reach 8.96 wt%, which exceed the target set by Department of Energy. The hydrogen adsorption energies with Gibbs free energy correction indicate that the adsorption of 6 H2 in (AlN)3, 8 H2 in (AlN)4 and 10 H2 in (AlN)5 is energetically favorable below 96.48, 61.43, and 34.21 K, respectively. These results are expected to motivate further the applications of clusters to be efficient hydrogen storage materials.