Activated carbons were prepared from carbonized phenolic resins by steam-activation via a pre-treatment, that is mixing of raw materials with Ca(NO3)(2)center dot 4H(2)O and acid-treatment after carbonization. This pre-treatment method has been reported as a method to produce mesoporous activated carbons. In the present work, three different novolak type phenol formaldehyde resins were synthesized under different molar ratio of formaldehyde (F) to phenol (P) (F/P = 0.3, 0.5, and 0.8) and were used as the precursors for activated carbons. It was confirmed that the pre-treatment method effectively enhances mesopores on activated carbons from synthesized phenolic resins prepared by F/P = 0.3 and 0.5, and their pore size distributions depended on the synthesizing conditions (F/P). Namely, it was found that by decreasing the F/P ratio of the precursor resin to the values lower than those usually adopted for commercial phenolic resin production, i.e., less than 0.5, activated carbons with developed mesopores could be obtained. The micropore developments of activated carbons were also influenced by F/P, and the influences were on the formation of pores larger than 0.40 nm. It was suggested that the thermoplastic nature of prepared resins (that can be changed by F/P) influence the porous properties of resultant activated carbons and the effect of the pre-treatments.