The pore structures of microporous carbon fibers having less-crystalline structures with different activation extents were examined by small-angle X-ray scattering (SAXS), N-2 adsorption, and density measurements. The different kinds of porosities were associated with observable quantities and a new concept of an inaccessible pore was introduced. Inaccessible pores include closed pores and ultrapores which are accessible by He at 303 K but not by N-2 at 77 K. The surface area of samples was determined by both the high-resolution a(5) plot of the N-2 adsorption isotherm and the Debye-Bueche plot of the SAXS profile. The surface area a(x) from SAXS was greater than that a(s) from N-2 adsorption. The difference a(x) - a(s) was associated with the volume and size of the inaccessible pores. The possibility for a general application of this inaccessible pore characterization was discussed.