Hyperbranched poly (urea-urethane)-functionalized multiwalled carbon nanotubes (MWNT-HPUs) have been synthesized by a one-pot polycondensation of tolylene 2,4-diisocyanate and diethanolamine in the presence of MWNTs terminated with multiple hydroxy groups. FT-IR, Raman, H-1 NMR, and C-13 NMR spectra reveal that the HPU trees are covalently grafted onto the MWNT surfaces. After a high density of HPU trees (83.5 wt.-%) is attached to the MWNTs, core-shell nanostructures with MWNTs as the core and the HPU trees as the shell are formed. A loose and uniform nanotube network can be observed by TEM, SEM, and AFM. The resulting MWNT-HPUs are soluble in polar solvents such as dimethylformamide, dimethylacetamine, 1-methyl-2-pyrrolidinone, and dimethyl sulfoxide.