Mesoporous activated carbon fiber (ACF) was prepared from a phenolic resin containing a small amount (0.1 wt%) of an organic nickel complex through carbonization and steam activation. Microporous ACF as reference sample was also prepared from a phenolic resin without any agent. In both cases, for the mesoporous ACFs and the microporous ACFs, the electric double layer capacitance in propylene carbonate containing 0.5 mol dm(-3) (C2H5)(4)NBF4 (0.5 M TEABF(4)/PC) was not proportional to the EET specific surface area. This is due either to the low permeability of nonaqueous electrolyte or the low mobility of the ions in the narrow micropores. However, the mesoporous ACF showed a higher double layer capacitance than the microporous ACF. This result suggests that the presence of many mesopores promotes the formation of an effective double layer or the transfer of ions in the micropore.