The peculiar features of a relaxational mode in liquid are described in conjunction with light scattering and optical Kerr effect experiments. It is shown that the relaxational mode shows a clear rise process in time response, indicating the limitation of its effective frequency range. It also has a dissipative nature which causes an apparent breakdown of quantum mechanical theory based on a canonical system. The nonexponential response of the relaxational mode in a picosecond range is discussed using Mori formula and is interpreted in terms of non-Markovian dynamics of the system. It is suggested that non-Markovian to Markovian crossover plays an important role to characterize the temporal/structural hierarchy in nature.