Molecular dynamics simulations have been carried out on benzene in aqueous solution and clathrate hydrate (structure LT cages) in order to compare the stability of the systems and rotational motions of benzene between 227 and 390 K. The total potential energy is 3-7 kJ mol(-1) lower in the clathrate hydrate than that in the aqueous phase as a result of the formation of more stable hydrogen bonds in the rigid host frameworks. The rotational relaxation times of benzene molecules in both liquid water and clathrate hydrate cages increase with decreasing temperature. At corresponding low temperatures, however, the relaxation time of benzene in the clathrate hydrate is a few times shorter than that in the liquid water, as found recently by NMR experiment. Thus, the anomaly in the relaxation time of benzene experimentally observed can be interpreted as a transition of environment due to the formation of clathrate hydrate in the low-temperature regime.