Numerical simulations are performed to study the characteristics of thermo-acoustic oscillation of a gas confined in a loop-type pipe with the section of short and narrow channels. In order to simulate the irreversible heat exchange between the gas and the pipe wall, the compressible mass, momentum and energy conservation equations are solved in the loop pipe as well as the heat conduction equations in the pipe wall. Spontaneous oscillations are obtained when the temperature gradient along the narrow channel becomes large. It is found that the onset temperature of simulated thermo-acoustic oscillation agrees with that of the stability analysis and increases when the effect of gravity is not taken into account.