A polyether-type polyurethane (PU) containing silver (Ag) nanoparticles (4–7nm, 1.51×10−3–1.13×10−2wt-%) was prepared by mixing the waterborne PU with the nanoparticle suspension, casting and drying at 60°C. The Ag nanoparticles were found to be well dispersed in PU. A significant increase in the thermal stability and mechanical properties of the polymer was demonstrated in the nanocomposite PU films, which was believed a result of induced crystallization in the presence of Ag nanoparticles. The biostability was tested in a rat subcutaneous model. After 19 days of implantation, the PU containing Ag showed enhanced biostability and lowered foreign body reaction. The effect of Ag on the stability of the PU polymer was even more remarkable over a wider range of particle contents than that of the gold nanoparticles previously studied.