The phenol-formaldehyde resins with various degrees of cross-linking were prepared by changing the formaldehyde/phenol molar ratios (F/P). The influence of the ratio (F/P) on the structure of the prepared resin was examined by using thermogravimetric analysis (TG) and FT-IR. It was found that an increase of the ratio (F/P) leaded to large oligomer and a promoted cross-linking. And the activated carbons were prepared from these resins with CO₂-activation in order to examine the influence of the resin structure on the pore structure of the prepared activated carbon. There is little difference between the activated carbon prepared from high F/P ratio resin and one from low F/P ratio in the specific surface area and pore volume. However, the pore size distribution of the activated carbon prepared from the low F/P ratio resin was sharper than that of the activated carbon prepared from the high F/P ratio resin. It was deduced that this difference was caused by the difference in the crystallite size.