The low-lying energy structures of small planar aluminum clusters Al_n (n = 3–6, 8–10), hydrogenated small planar aluminum clusters Al_nH_m (n = 3–8, m = 1–2) and the lowest-energy structure of Al_nH_m (n = 6–10, m = 0–2) are determined by density functional theory (DFT) calculations. Many stable planar structures have been found; some are consistent with the reported ones, and some are new configurations. The preservation of planar cluster structures has been observed during the dissociative adsorption of H₂.Hydrogen is adsorbed at different positions on planar aluminum clusters. Dissociative adsorption configurations of the planar structure and lowest-energy structure experienced a decrease in hydrogen adsorption energy with an increase in cluster size. Among the clusters we calculated, Al₄H₁ and Al₄H₂ have the highest HOMO-LUMO gap, indicating that they may be more abundant than other clusters. The geometric structure and electronic properties of these clusters are also discussed.