Abstract: Two dimensional small angle X-ray scattering (2D-SAXS) was used to characterize the detailed pore structure inside carbon fibers from different azimuth angles. Chinese PAN carbon fibers (MHS) and Japanese carbon fibers (T300 and T800) were investigated and compared by assuming ellipsoidal pores. The pore size of MHS was found to be between those of T300 and T800. Meanwhile, the length/diameter ratio (t) of pores of MHS was the smallest among the three kinds of fibers. Compared to MHS, T300 had a large amount of pores with sizes less than 9.1nm and larger than 15.4nm. T800 mainly contained small pores with size less than 2nm along the short axis. The surface fractal dimension indicated that the interface between pore and solid carbon in MHS was rougher than for T300 and smoother than for T800. The large deviation from the Porod law indicated that there was an electron density fluctuation within the solid matrix of the carbon fibers. This electron density fluctuation was possibly due to the cavities between microcrystallines in the carbon matrix. Among the three kinds of fibers the weakest density fluctuation occurred in T800 and the density fluctuation amplitude F and dimension n decreased from MHS to T300 to T800. Therefore, T800 had the most favorable pore arrangement to improve mechanical strength.