We carry out detailed computational investigations of the decomposition and dissolution processes of a cellulose I-beta fiber in the ionic liquid (IL) solvent, [C(2)MIm][OAc]. First, we investigated the properties of the interactions between cellulose chains in the cellulose fiber, including interchain H-bonds and stacking interactions, with the quantum and molecular mechanics (QM/MM) methods, employing a microscopic solvent model. From the calculation results, it is indicated that interchain interaction energies are largely influenced in the axial direction by the solvent effects of the IL and that the degree of interactions depends on the site of the glucose unit, compared to that in the equatorial (parallel) direction. To further investigate the impact of the IL on intrachain H-bonds and its relation to interchain interaction, we perform molecular dynamics (MD) simulations. Our results indicate that it is difficult to disrupt a strong three-dimensional H-bond network in the cellulose fiber at room temperature, even with ILs. On the other hand, the total number of H-bonds in the cellulose fiber continues to decrease from the beginning of the dissolution and decomposition processes in the IL at 400 K. The results indicate that the number of inter- and intrachain H-bonds reduces sequentially and that intrachain H-bond breakage inside the cellulose fiber proceeds prior to interchain H-bond disruption. Also, it is shown that the breakage of interchain H-bonds starts playing an important role in enhancing the separation of cellulose chains from each other. On the role of anions in the dissolution and decomposition processes of the cellulose fiber in the IL, our results indicate that the formation of H-bonds between [OAc](-) anions and a cellulose chain is facilitated by the intercalation of [OAc](-) anions into the cellulose fiber and that, in particular, the breakage of intrachain H-bonds in cellulose chains due to [OAc](-) anions proceeds prior to that of interchain H-bonds. On the role of cations, it is shown that [C(2)MIm](+) cations could interact with the cellulose and stabilize detached cellulose chains due to the stacking effect through the van der Waals interaction, in particular, within the first solvation shell of a cellulose chain in ILs. Our results suggest that the enhancement of the flexibility of rigid cellulose chains triggered by the breakage of intrachain H-bonds due to anions starts decomposition processes accompanied by dissolution processes due to the intercalation of cations, synergistically, and, then, both dissolution and decomposition processes are executed simultaneously.