Experimental results are presented for the incorporation of carbon nanotube in polycrystalline MgB/sub 2/ superconductor based on X-ray diffraction and transmission electron microscopy measurements. Electron microscopy studies show that nanotubes are embedded into the MgB/sub 2/ matrix with a fraction of nanotubes found to be unreacted and entangled. In contrast, magnetization measurements indicate a change in the critical current density with the length of nanotubes and not with their outside diameter. This implies that longer nanotubes tend to entangle, preventing their homogenous mixing with MgB/sub 2/ and dispersion. Overall, carbon nanotube doping of MgB/sub 2/ enhanced the critical density and depressed the critical temperature.