Ammonia-borane (AB, NH3BH3) has been known as a potential hydrogen storage material due to its high gravimetric capacity. However, there are some barriers to overcome for AB to be realized as a practically useful material. Among them, maximization of the utilization of H2 in AB and regeneration of the used AB are the main problems, which are related to the forms of products after the first hydrogen extraction. After the first H2 extraction is done, the products have various forms including monomer, cyclic oligomers, and polymers. The variety of products prohibits the further extraction of H2 and regeneration of the used fuel. Therefore, the fuel should be designed to produce simply one or two forms of products of the first H2 extraction. We investigated the stability of monomers, dimers, and trimers of alkyl substituted aminoborane (RNHBH2) using density functional theory calculations. When no R group is present, cyclization of monomers is an exothermic process so that the cyclic trimers could be made preferentially. If the R groups are substituted, the general trend does not change and cyclic dimers and trimers are more stable than corresponding monomer. Phenyl ring substitution weakens the bond strength of B-N bonds and this makes the stabilization of cyclization smaller.