The expression of the correction factor in nucleation theory is derived by extending the method Reiss used recently. is the factor appearing in the number of critical nuclei (formed as a vapor condenses into liquid drops) as a correction to the conventional theory. It is shown that=p l / g /kT, wherep l is the pressure of the liquid phase inside the drop, g , is the volume per molecule in the vapor phase,k is the Boltzmann constant, andT is the absolute temperature. The difference between this and Reiss's expression isp l , which replaces hisp g (the vapor pressure in equilibrium with the drop). The derived in this paper is compatible with the expression= g / l ( l is the molecular volume in the liquid phase) previously proposed by the present author.