Weight-average molecular weight ðM w Þ and intrinsic viscosity ½h of six fractions for benzyl konjac glucomannan (B-KGM) were determined by laser light scattering and viscometry, and the Mark-Houwink equation was established to be ½h ¼ 2:08 £ 10 23 M 0:87 w ðcm 3 g 21 Þ in the M w range of from 4.42 £ 10 4 to 33.74 £ 10 4 in N,N-dimethylformamide (DMF) at 25 8C. Based on current theories for a wormlike chain, the conformational parameters of B-KGM were found to be 1284 nm 21 for molar mass per unit contour length ðM L Þ; 9.7 nm for persistence length ðqÞ; and 22.2 for characteristic ratio ðC 1 Þ; suggesting that B-KGM existed as a semi-stiff chain with relatively large steric-hindrance in DMF. By incorporating the semi-stiff B-KGM of different M w into castor oil-based polyurethane, a series of semi-interpenetrating polymer networks (semi-IPN) films, coded as UB, were successfully prepared. The effects of M w and chain conformation of B-KGM on miscibility and properties of the resulting UB films were investigated by using Fourier transform infrared spectroscopy, dynamic mechanical thermal analysis, ultraviolet spectrophotometer, and measurements of crosslink density and mechanical properties. The results indicated that the light transmittance of the UB films increased from 77 to 89% with a decrease of B-KGM M w from 33.74 £ 10 4 to 4.42 £ 10 4 due to intimately interfacial adhesion between the two phases. As the B-KGM M w decreased, the tensile strength of the UB films decreased from 15 to 5.4 MPa, whereas the elongation at break increased from 54 to 120%, resulted from that the stiff B-KGM molecules of lower M w could easily penetrated into PU phase to hinder the formation of PU network. Miscibility between PU and lower M w B-KGM was obviously higher than that between PU and higher M w B-KGM, due to the relatively large specific surface provided by lower M w B-KGM similar to 'nanoparticle'. Meanwhile, miscibility between PU and B-KGM was higher than that between PU and nitrocellulose or nitro-KGM, indicating that the effect of the chain stiffness of the natural polymers on the miscibility of the semi -IPN systems not be negligible.