Carbon nanotubes (CNTs) are different from other porous substrates such as activated carbon due to their high external surfaces. This structural feature can lead in some uncertainties in the results of nitrogen adsorption analysis for characterization of CNTs. In this paper, the results of microscopic analyses and nitrogen adsorption method for characterization of carbon nanotubes were compared. Five different types of CNTs with different structures were either synthesized or purchased. The CNT samples were characterized by high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and N-2 adsorption analysis. The comparisons between the results from the microscopic analyses and N-2 adsorption showed that the total pore volume and BET surface measurements include the internal and external porosity of CNTs. Therefore, the interpretation of N-2 adsorption data required accurate TEM analysis. In addition, the evaluation of pore size distribution curves from all CNT samples in this study and several instances in the literature revealed the presence of a common peak in the range of 2-5 nm. This peak does not explain the inner pore size distribution. The presence of this common peak can be attributed to the strong adsorption of N-2 on the junction of touched and crossed nanotubes. (C) 2017 Elsevier B.V. All rights reserved.