A development of micropores of $CO_2$activated isotropic carbon fibers from TEM was observed. It was observed that the micropores of activated carbon fibers(ACFs) were consisted of slit-shaped pores(SP) and cylinder-shaped pores(CP). The SPs were formed between two parallel-carbon layers, and the CPs were formed at a place which is connected polygonally by more than two carbon layers. It was shown that the CPs of the ACFs were developed at high degree of burn-offs and at high activation temperature. The pore size distribution of the best ACF, which was observed at a highest value of specific surface area(3,495 $m^2$/g), showed a continuous distribution in the range of about $4∼l5AA$, and the median pore size was 6.7$AA$. The super-high specific surface area of ACFs was found to be due to that the SPs were connected with a maximum size of 7∼8$AA$ continuously, It is possible that the SPs should be formed in the ACFs in order to show super-high SSA.