Since a glycerol molecule has three active sites, two ${alpha}$ and one ${eta}$ hydroxyl groups; it undergoes condensation by releasing water molecules to produce linear, nonlinear and heterocyclic oligomers. The Gibbs free energy (G), enthalpy (H) and internal energy (E) of 7 diglycerol, 15 triglycerol and 23 tetraglycerol isomers were calculated at B3LYP level of theory using 6-311++G(d, p) basis set, in both gas and aqueous phases. Linear oligomers, ${alpha}{alpha}$-diglycerol, ${alpha}{alpha}$, ${alpha}{alpha}$-triglycerol and ${alpha}{alpha}$, ${alpha}{alpha}$, ${alpha}{alpha}$-tetraglycerol, were found to be the most stable oligomers in aqueous phase. It was found that the stability of cyclic oligomers decreases as the size of their rings increases. Cyclic oligomers are produced by dehydration of the acyclic ones which is an endothermic reaction while its ${Delta}G$ is negative. The dehydration reaction is less endothermic in aqueous phase.