A series of semi-interpenetrating polymer networks (semi-IPNs) films were prepared from 20 wt %, of benzyl amylose (BA) of different M-w and castor oil-based polyurethane (PU) in N,N-dimethylformamide (DMF). The weight-average molecular weight (M-w), and radii of gyration ((1/2)(z)) of benzyl amylose were determined by laser scattering measurement, and the results suggested BA was in a compact coil conformation in DMF. Furthermore, the properties and miscibility of the polyurethane/benzyl amylose (PUBA) films were studied by scanning electronic microscopy, differential scanning calorimetry, dynamic mechanical thermal analysis, ultraviolet-visible spectrophotometer, and tensile testing. The PUBA films possessed much higher optical transmittance and tensile strength than the pure PU film regardless of the molecular weight of BA, but lower values of elongation at break were observed. With decreasing of the BA M-w from 9.24 x 10(5) to 2.69 x 10(5), interestingly, the elongation at break of the films increased from 135 to 326%. This might be ascribed to the decrease of crosslinking density of PU networks and the enhancement in freedom of the molecular motion. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1299-1305, 2010