Polyurethane-nitrolignin (PUNL), a new network polymer, was synthesized from a castor oil based-polyurethane (PU) prepolymer and nitrolignin (NL) with a weight-average molecular weight of 20.6 x 10(4) and a content of 1.4-10%. The structure and miscibility of PUNL films prepared by solution casting were investigated by infrared spectroscopy and transmission electron microscopy. The results indicated that PUNL2 film, which had a 2.8% NL content, was the most miscible, and its tensile strength (sigma (b)) and breaking elongation (epsilon (b)) were 2 times higher than that of PU film. The crosslink densities of PUNL films increased with the increase of NL content until about 3%, similar to the variety of the mechanical properties. Thermogravimetric analysis revealed that the thermal stability of PUNL films was slightly higher than that of PU. Covalent bonds occurred between PU prepolymer and the NL in the PUNL films, forming crosslink networks, which resulted in the enhancement of mechanical properties and thermal stability. NL has a far higher reactivity with PU than nitrocellulose.