Multiwalled carbon nanotubes (CNTs) were modified in low pressure capacitively coupled discharges (13.56 and 27.12 MHz) in Ar/NH3 and oxygen-containing gas mixtures. A direct functionalization by nitrogen groups was not possible but 1-3 percentage of carbon-oxygen bonds increased with the total oxygen content on the expenses of sp(2)C almost independently on the plasma conditions. The plasma modified CNTs were used as fillers for polyurethane (PU) composites prepared by in situ polymerization. The composites were investigated by depth sensing indentation that revealed the existence of surface harder layer caused probably by different polymerization process in the bulk and at the surface that was in a contact with air. The significant improvement of the hardness and elastic modulus was observed when plasma-modified CNTs with high amount of oxygen were added to PU. It also improved the creep resistance of the PU, whereas the ability to recover from a deformation, i.e. anelastic recovery, did not change much. (c) 2012 Elsevier B.V. All rights reserved.