It has been reported that MgB2 lost its superconductivity when doped with 40 at.% Al. Here, we report on the influence of carbon nanotubes (CNTs) on the structure and superconductivity of MgB2-CNTs composites. The lattice parameters of MgB2 with 10 at.% CNTs are a = 0.30815 nm and c = 0.35322 nm, slightly different from those of the pure MgB2, which indicates that carbon atoms may have substituted boron atoms in MgB2. The transition temperature of the samples decreases as the CNTs content increases. However, the samples added with about 40 at.% CNTs still have a transition temperature of 24 K. When added with bulk CNTs (between 20 and 40 at.%), the samples present a semiconductor behavior in the high-temperature region (above 40 K) and have superconductivity at low temperature; (below 32 K). (C) 2002 Elsevier Science B.V. All rights reserved.