We have reported that a star-like network structure with lignin as center in polyurethane resulted in the simultaneous enhancement in mechanical strength and elongation. In this study, we aimed to enhance the mechanical properties of waterborne polyurethane by adding nitro-lignin (NL) to form analogical star-like network. It was found that the resultant material has optimal mechanical performances when the NL content is 3.0 wt %, i.e., its apparent mechanical strength and elongation increased by about 80% at one time. Especially, its real mechanical strength reaches 71.3 MPa at this time, which is 3.6-fold over that of neat waterborne polyurethane material. The simultaneous enhancements in strength and elongation are attributed to the forming of star-like network in the composites. The stiffness of lignin improved the mechanical strength, while the entangling and crosslinking in polyurethane component increased the elongation. However, higher NL loading and lower grafting level induced the forming of supramolecular NL aggregates, and hence greatly inhibited star-like network, resulting in lower mechanical strength and elongation. However, the Young's modulus of the material is enhanced with an increase of rigid supramolecular aggregates. (C) 2008 Wiley Periodicals, Inc.