Polyacrylonitrile(PAN)-based activated carbon fibers (ACFs) prep ared by different activation methods were characterized using low-temperature nitrogen adsorption over a wid e relative pressure from 10-6 to 1. With the aid of nitrogen adsorption, the po re structure parameters and pore size distribution were calculated using differe nt models. By these analyses,the pore development and the dominant pores of samp les prepared by various activation methods could be clearly observed. For steam -activated series of samples, with increasing activation the values of specific surface area increase and the pores are gradually widened. The dominant pores c hange from micropores to mesopores and some macropores are developed when the de gree of activation is very high. For air-activated series of samples, only the ultramicropores are developed seriously. The dominant pores can be thought to be ultramicropore. For KOH-activated series of samples, the dominant pores can be thought to be supermicropores. Moreover, the adsorption measurement could provi de profound insight into the structural heterogeneity of the ACFs.