Abstarct: In this study, the structural and electronic properties of graphene (Gr) and graphene-like hexagonal boron-nitride (h-BN) nanosheets decorated with transition metel clusters (Ptn, Pdn, and Irn; n = 5, 6) for hydrogen storage were investigated. This study was conducted using density functional theory (DFT) with GGA approach implemented in OpenMX code. The adsorption of the TMn clusters (n=5 and 6) on the most stable adsorption sites of nanosheets of the Gr (bridge site), and the h-BN (N-top site) was studied. The electronic charge analysis showed that the charge transmitted from the TM atoms to the C atoms of the graphene sheet. The results of the partial density of states (PDOS) indicated appearance of a peak near the Dirac point due to the TM: d states. The results obtained from study of adsorption of the TMn clusters on the h-BN sheet showed charge transfer from the TM atoms of clusters as well as from B atoms to N atoms of the h-BN sheet due to the difference in electro-negativities. Investigation of the band structure revealed that the adsorption of the TMn clusters on the h-BN sheet create new states around the Fermi level and greatly reduce the band gap of the h-BN sheet. Finally, the possibility of adsorption and storage of the hydrogen on the optimized BN/TMn and Gr/TMn structures was studied. The results showed that the d orbital of the TM atom acts as an adsorption centers for the hydrogen molecules and polarizes the H2 molecules and leads to the adsorption of H2 molecules. The hydrogen storage capacity was calculated for the BN/TMn and Gr/TMn structures. The results obtained revealed that according to the adsorption energy values and hydrogen storage capacity, BN/Pd6, and Gr/Pd6 systems are the most suitable structures among the studied structures for hydrogen storage.