Equilibrium and nonequilibrium molecular dynamics (MD) simulations have been performed in order to account for anomalous thermal conductivity of clathrate hydrates. Evaluated thermal conductivities of clathrate hydrates are lower than those of ice from both types of MD simulations. The evaluated values agree qualitatively with experimental observation. Its anomalous temperature dependence is also reproduced. It is found that those unique properties of clathrate hydrates arise mainly from encaging of guest species and that guest Xe atom even in large cage couples strongly with the host lattice and vibrations relevant to Xe have a large anharmonic contribution. Those modes with large anharmonicity interact with other modes and scatter phonons. This leads to a small thermal conductivity. Increasing temperature changes the "effective" potential energy curves of guests in a different way from a the normal crystal. Softening of the modes relevant to guests is suppressed, which is unique in clathrate hydrates reducing anharmonicity of the low frequency modes coupled with guest motions. It is concluded that the most likely mechanism for the anomalies is the resonance scattering model among three accounts proposed so far.