The nanostructure and porosity of activated carbon fibers (ACFs) prepared by physical activation with CO2 and by chemical activation with H3PO4 of the highly ordered polymer poly(m-phenylene isophthalamide) have been investigated and compared by means of scanning tunneling microscopy (STM), scanning electron microscopy (SEM), and gas adsorption measurements. In general terms, both types of activation led to porous carbons with similar nanometer-scale structure, which consisted of relatively ordered and homogeneous arrays of platelets below 10-nm wide, the porous structure being mainly comprised by the network of narrow trenches present between neighboring platelets. This similarity was attributed to the influence of the crystalline structure of the polymeric precursor, which should favor a homogeneous, uniform transformation of the polymer into the final carbon material. Such influence was only lost in chemical activation with the use of very large amounts of activating agent. A comparison of samples before and after physical activation allowed a direct identification of the local areas where gasification (activation) took place. For chemical activation, the STM measurements suggested that porosity was developed at a lower temperature than the highly cross-linked nanographitic structure of the final ACF. This result was discussed in terms of the thermal transformation mechanism of the precursor polymer into a carbonaceous solid in the presence of H3PO4.