Heat treatment and acid oxidation of multi-walled carbon nanotubes (MWNTs) were conducted in this study. Fractal analysis was carried out to study the surface properties of the MWNTs after modifications. The results of the present work show that there was a contraction in the hysteresis loop at P/P-o around 0.80, which differentiated the capillary condensation in the internal pores from that in the aggregated pores forming by the entangled MWNTs. The maximum of the pore size distribution (PSD) curves in the mesopore range did not shift after modification because it described the major inner diameters of the MWNTs. A smooth and ordered surface could be formed after heat treatment, but a much rougher and less homogeneous surface was produced on acid-treated MWNTs. Moreover, an excellent linear increase in the surface fractal dimension (d (s) ) values was observed as the micropore surface area (S-mi) increased, which implies that the generation of micropores would contribute to the surface roughness.