The search for materials from renewable sources such as vegetable oil-based polymers, polyurethane based on castor oil represents a promising alternative in researches. Using castor oil polyurethane as matrix for composites reinforced with lignin and cellulose from sugarcane straw is in tune with economical and environmental interests. The objective of this work was the evaluation of the changes on the structure and thermal behavior of castor oil polyurethane composites reinforced with lignin and cellulose from sugarcane straw. The cellulose and lignin were extracted from the straw by acid hydrolysis and then they were milled. For the production of the polyurethane, the mass reason between polyol (castor oil) and diisocyanate was 1.5:1.0. Reinforcement of the matrix was done changing the concentration of cellulose and lignin. SEM micrographs of fractured surfaces of the obtained composites and thermal analysis (TGA) were done. No lignin could be seen in SEM micrographs, what suggests a blend material behavior. The addition of cellulose fibers (30%) and lignin (40%) to the polyurethane matrix has caused an increase of the stiffness in the resulting composite when compared with the pure matrix. Above these rates, stiffness decreases. The presence of lignin or cellulose fibers in polyurethane matrix seems to increase mechanical properties of the composite, and also introduce better thermal stability than the pure matrix. Its presence might also bring other significant synergetic properties to the final composite, what has currently been studied. At the same time, the blend behavior of Polyurethane-Lignin composites has been investigated.