Carbon nanotubes have attracted attention around the world because of their high sorption capacity for hydrophobic organic compounds (HOCs); however, the bioavailability of HOCs sorbed on carbon nanotubes to bacteria is not well known. In the present study, (14) C-labeled and unlabeled phenanthrene was spiked onto four kinds of multiwalled carbon nanotubes (MWCNTs) and aged for 60 d, and the mineralization of phenanthrene sorbed on MWCNTs was studied. The mineralization efficiencies of phenanthrene sorbed on the four kinds of MWCNTs were 2.38, 8.84, 23.88, and 31.47%, respectively, after incubation for 35 d, which were positively correlated with the sum of fast and slow desorption fractions of phenanthrene (p < 0.05). The authors found that the mineralization efficiencies agreed well with the MWCNT properties; the increasing specific surface area and mesopore and micropore volume of MWCNTs caused a significant decrease in the mineralization efficiency of phenanthrene (p < 0.05). After incubation for 35 d, the density of bacteria had a significant positive correlation with both the desorption and the mineralization efficiencies. The authors inferred that, although MWCNTs might have toxic effects on the bacteria, the biodegrading ability of bacteria might not decrease significantly under the influence of MWCNTs with low surface area.