AbstractThe thermostability and mechanical properties of ultra-low density fiberboard (ULDF) were improved with the different content of silica sol. Microstructure and properties of ULDF were tested using scanning electron microscope (SEM), thermogravimetric analyzer (TGA), and microcomputer control electronic universal testing machine. The microstructures and the relative density of ULDFs were different with changes in Si sol content. The TGA results showed that the residual weight of ULDF was increased with the increasing content of silica sol and that the thermostability of ULDFs was improved. The modulus of rupture (MOR), modulus of elasticity (MOE), and the internal bond strength (IB) of ULDF were significantly improved from 0.12, 10.86, and 0.020 MPa to their maximum values of 0.23, 23.36, and 0.031 MPa while 4% silica sol was added.