Abstract: We have prepared waterborne polyurethane(WPU) nano-SiO₂ composites by in situ polymerization,during which modified silica sol was first mixed with the N-methyl-pyrolidone(NMP),and then polyme-rization was carried out with the addition of isophorone diiocyanate(IPDI),polycaprolactone(PCL) dimethylol-propionic acid(DMPA) and dibutyltin dilaurate.Effects of nano-silica content on the thermal and mechanical properties of WPU/nano-silica composites were investigated by thermogravimetric analysis(TGA),differential scanning calorimetry(DSC),transmittance electron microscopy(TEM) and dynamic mechanical analysis(DMA).The experiment results show that the nano-silica particles were well dispersed in the WBPU solution when the silica content was lower than 3.0%,a wider T_g gap and higher microphase separation degree between soft and hard segment were observed for WPUNS,and the thermal degradation temperature of the composite was higher than that of a pure WPU at temperature over 50 ℃.Besides,as the nano-silica content was increased from 0 to 0.5%,the elongation at break and water swell were decreased from 550% to 430% and 16.8% to 4.5%,respectively,while,the tensile strength was increased from 3.4 MPa to 5.95 MPa.As the nano-silica content was increased to 2.5%,the elongation at break and water swell decreased to 210% and 2.3%,and the tensile strength increased to 7.58 MPa.