Various isomers of Si4H2 cluster have been investigated with ab initio molecular orbital and density functional theory (DFT) calculations. Nine local minimum isomers on the potential energy surface have been obtained with both Moller-Plesset perturbation theory (MP2) and DFT methods. The Si-4 frame is slightly distorted by the dissociative adsorption of H-2 on it. The most stable isomer of Si4H2 is a classical structure with both hydrogen atoms bonded to a single silicon atom. The nonclassical H-bridged structures are also found in the calculations, but predicted to be less stable than the nonbridged structures energetically. The formation of the most stable isomer of Si4H2 from Si-4 and H-2 is proven to be a two-step process and exothermic. The first step is the dissociative adsorption of H-2 on Si-4 cluster by overcoming an energy barrier of 19.27 kcal/mol, and the second step of conversion from the intermediate to the product will readily proceed with a barrier of only 0.53 kcal/mol.