Molecular dynamics simulations are used to study the mechanism of guest replacement of CH4 hydrate with CO2. The well-known OPLS-AA and TIP4P potential models are used for the interactions between guest-guest and water-water species, respectively. The simulations are performed on a combination of 4 x 4 x 4 unit cell replica of fully-occupied structure I hydrate and CO2 gas phase. The simulation results confirm that CH4 is released from its hydrate and enter into the gas phase by the replacement with CO2, which has been testified by in situ Raman spectroscopic experiments. Without the hydrate dissociation, CH4 molecules are difficult to run out of while CO2 to penetrate into the interior cages of the hydrate block because of the barrier of the cage walls constructed by hydrogen-bonding network of water. It is conjectured that the replacements of CH4 hydrate with CO2 will spend long time without the dissociation, or at least slightly melting of the hydrate crystals once time. The replacement process may be divided into three steps: the cages are broken firstly, then CH4 molecules run out of the cages and meanwhile CO2 molecules enter into the void cages and occupy them in stead. (C) 2011 Elsevier Ltd. All rights reserved.