SessionsGrowth process of methane hydrate from D2O ice powder was measured under pressurized CH4 gas by in situ neutron powder diffraction.We used HRPD and HERMES [1] at JRR-3 in Japan.To avoid melting ice, the CH4 gas was gradually applied at 240 K as an isothermal process.The diffraction peaks of methane hydrate were observed at 2 MPa.The pressure was kept at 2 MPa for several hours.After that, the pressure was increased until 6 MPa.However, the growth rate of methane hydrate did not change.Accordingly, the pressure did not affect the growth rate under the sufficient pressure for the crystal growth.Following, the temperature was changed under nearly isobaric process (6-7 MPa).When temperature was changed from 265 K to 270 K, an increase of the growth rate was observed.At 275 K, the D2O ice was melted and the growth rate drastically increased.After 4 hours, almost all of the D2O ice was changed to methane hydrate.As a result, temperature is more effective parameter for the crystal growth of methane hydrate.Moreover, we will discuss the size effect of ice grain.