The dependence of starting materials and their initial grain sizes on the formation of gases(H2, CH4, C2H6 and C3H8) during serpentinization was investigated by conducting hydrothermal experiments at 311°C and 3 kbar on olivine and peridotite with initial grain sizes ranging from <30 to 177 ?m. Hydrocarbons(CH4, C2H6 and C3H8) were produced from reaction between dissolved CO2 in the starting fluids and H2 formed during serpentinization, which were analyzed by Gas Chromatography. It was found that olivine serpentinization produced much less H2 and CH4 compared with those after peridotite alteration, while their C2H6 and C3H8 were identical. For example, for olivine with initial grain sizes of <30 ?m, the amounts of H2 and CH4 were 79.6 mmol/kg and 460 ?mol/kg after 27 days, respectively. By contrast, the quantities of H2 and CH4 produced in experiment on peridotite with the same run duration were much larger, 119 mmol/kg and 1300 ?mol/kg, respectively. This indicates that spinel and pyroxene in peridotite may increase the amounts of H2 and hydrocarbons, possibly due to the catalytic effect of aluminum released by spinel and pyroxene during serpentinization. Moreover, the production of H2 and hydrocarbons is negatively correlated with initial grain sizes of the starting material, with smaller amounts of H2 and hydrocarbons for larger initial grain sizes, indicating that the kinetics of serpentinization influences the formation of H2 and hydrocarbons, possibly because of the lack of catalytic minerals for the starting material with larger grain sizes. This study suggests that olivine cannot completely represent peridotite during serpentinization, and that H2 and hydrocarbons in hydrothermal fields near the mid-ocean ridge may be produced in a very long period of serpentinization or the presence of catalytic minerals due to large grain sizes of ultramafic rocks. The dependence of starting materials and their initial grain sizes on the formation of gases (H2, CH4, C2H6 and C3H8) during serpentinization was investigated by conducting hydrothermal experiments at 311 ° C and 3 kbar on olivine and peridotite with initial grain sizes ranging from < 30 to 177? M. Hydrocarbons (CH4, C2H6 and C3H8) were produced from reaction between dissolved CO2 in the starting fluids and H2 formed during serpentinization, which were analyzed by Gas Chromatography. It was found that olivine serpentinization produced much less H2 and CH4 For example, for olivine with initial grain sizes of <30 μm, the amounts of H2 and CH4 were 79.6 mmol / kg and 460 μmol / kg after 27 days , respectively. By contrast, the quantities of H2 and CH4 produced in experiment on peridotite with the same run duration were much larger, 119 mmol / kg and 1300? mol / kg, respectively. This indicates that spinel and py In addition, the production of H2 and hydrocarbons is the negatively correlated with initial grain sizes of the starting material, with the smaller part of the amount of H2 and hydrocarbons, possibly due to the catalytic effect of aluminum released by spinel and pyroxene during serpentinization. amounts of H2 and hydrocarbons for larger initial grain sizes, indicating that the kinetics of serpentinization influences the formation of H2 and hydrocarbons, possibly because of the lack of catalytic minerals for the starting material with larger grain sizes. This study suggests that olivine can not completely represent peridotite during serpentinization, and that H2 and hydrocarbons in hydrothermal fields near the mid-ocean ridge may be produced in a very long period of serpentinization or the presence of catalytic minerals due to large grain sizes of ultramafic rocks.