To examine the characteristics of the explosion of city gas, the concentration of oxygen was changed with the change of initial pressure. From the result of the experiment, as the concentration of oxygen was low, the explosion limit became narrow and the minimum concentration of oxygen for the explosion was $12\%$. Furthermore, As the increase of the initial pressure, explosion ranges were a little increased. And as the change of the initial pressure, the maximum explosion pressure were $6.3 kgf/cm^2{cdot}g,;12.7 kgf/cm^2{cdot}g$ and the maximum pressure rising velocity were $245.63 kgf/cm^2/s,;427.88 kgf/cm^2/s$.