Two kinds of the porous silicas with uniform and cylindrical pores were prepared from calcined Na4SiO4, and cationic surfactants ([C16H33N(CH3)(3)]Cl and [C18H37N(CH3)(3)]BT) In this paper, we call these porous silicas PS-C16 and PS-C18. PS-C16 was calcined in vacuo at 873-1273 K in order to prepare a series of porous silicas having different pore sizes. All nitrogen adsorption isotherms for a series of the porous silicas show no hysteresis in the adsorption and desorption branches. The nitrogen adsorption isotherms were analyzed by the t method, and the pore surface area (S-p), the pore volume (V-p), and the core volume (V-c) were determined, The pore radius (r(p)), the core radius (r(c)), and the adsorbed thickness on the pore wall (t(pore)) were computed from the measured values of S-p, V-p, and V-c. We can determine the relationship between the Polanyi adsorption potential and the core radius in the range of r(c) = 0.6-1.2 nm. On the basis of the present data, we propose a new method of calculating the pore size distribution in the range of r(p) = 1-2 nm,