A mathematical model for an unsteady flame is established and applied to hydrogen/air premixed flames propagating spherically in a closed vessel. The model proposed in this investigation comprises the full reaction scheme for hydrogen combustion. It contains twenty-one elementary reactions and nine chemical species. The predicted burning velocities of hydrogen/air premixed flames with various combustion conditions are in good agreement with the experimental ones. The linearity between the mass fraction of the burnt gas and the pressure proposed by Lewis and von Elbe is reproduced and verified to be valid, especially at the early stages of combustion. The reaction mechanism of hydrogen/air premixed flames propagating unsteadily is fairly similar to that of the steady flames. It can be divided into two parts, the low-temperature mechanism and the high-temperature mechanism. The boundary temperature is around 1000K in all the flames. In a low-temperature region, the necessary amount of thermal energy to heat the combustible gas mixtures up to the boundary temperature is supplied in the form of chemical energy. It is transferred by the hydrogen atoms diffused from the flame fronts, and released mainly through the recombination reaction with oxygen
