Castor oil and microcrystalline cellulose were employed as biomass feedstock to produce bio-based polymer composites with increased tensile strength. The polymer composites were prepared by curing castor oil with 4,4′-methylenebis (phenyl isocyanate) (MDI) in the presence of microcrystalline cellulose (MC) or modified MC (MMC). The MMC was prepared by reacting MDI with MC to introduce isocyanate groups to the surface of MC. X-ray diffraction spectra suggested the good retention of the crystalline structure of MC or MMC in the composites. SEM analysis showed the well dispersion of MC or MMC in the composites. All of these factors are critical to reinforcing the composites. Mechanical tests of the composites revealed that the reinforcing effect of MMC was more significant than MC at high cellulose content such that the highest tensile strengths of 4.87 MPa was obtained for the composite containing 43% (wt) of MMC. That is almost 5 times higher than that of neat castor oil-based polyurethane.