Semiinterpenetrating polymer networks (semi-IPNs) were synthesized from castor oil-based polyurethane (PU) and 20 wt % nitrolionjac glucomannan (NKGM) with weight-average molecular weights (M-w) from 2.86 x 10(4) to 14.1 x 104. Results from the dynamic mechanical analysis, differential scanning calorimetry, wide-angle X-ray diffraction, and ultraviolet spectrometer showed that the semi-IPNs have a single broad alpha -relaxation peak, one glass transition, and higher optical transmittance (Tr) in the wavelength range of 400-800 nm than PU, suggesting good miscibility in the range of NKGM M-w used. Noted that tensile strength (sigma (b)) of the semi-IPNs films was much higher than that of films PU and NKGM, and the Tr, sigma (b), and breaking elongation (epsilon (b)) obviously increased with decrease of NKGM M-w from 8.44 x 10(4) to 4.75 x 10(4) (Tr = 88% at 800 nm, sigma (b) = 34 MPa, epsilon (b) = 110%). This indicated that the NKGM with relatively lower M-w plays an important role in plasticizing, accelerating cure, and enhancement of intermolecular interaction between two polymers in the semi-IPNs films. The semi-IPNs system provided a novel way for modification and exploitation of natural polymers such as polysaccharides.