A thermodynamic stability analysis of microbubbles in an enclosed volume of a volatile liquid-noncondensable gas solution was performed. The stability conditions for the formation of the bubbles and the relationship between the equilibrium bubble radius and the equilibrium bubble number were investigated. The calculation results show that even if the saturation vapor pressure of a volatile liquid is higher than the pressure of the liquid phase, stable noncondensable gas bubbles appear in a closed system. Under such a condition, if the initial pressure of the noncondensable gas, which is proportional to the total number of moles of a noncondensable gas in the system, increases, stable bubbles appear at a lower temperature. At a fixed parameter of h(0), which is defined as the ratio of the saturation vapor pressure of the volatile liquid minus the pressure in the liquid phase to the initial pressure of the noncondensable gas, the equilibrium bubble radius increases with a decrease of the equilibrium bubble number.