Previously, different characteristics such as curing, morphology, miscibility and swelling have been evaluated in studies of polyurethane-lignin polyblends. In the present paper the mechanical properties of the same polyblends are investigated and compared with those of polyurethane filled with some mineral fillers. The effect of these fillers and lignin on the polyurethane sealant curing kinetics and the effect of their packing characteristics on the mechanical properties are considered and discussed. The research showed that the addition of mineral fillers or different kinds of lignins increases the Young modulus. The enhancement of the mechanical characteristics is more evident for Sillitin/Titanox (clay/TiO2) filled polyurethane than for lignin filled polyurethane. This mineral filler combination increases both stress and strain, which is attributed to its finer average particle size.