Acidic and alkaline silica sols were used to impregnate wood samples. Then the wood sample’s microstructure was observed using an SEM. Bending properties and thermal degradation properties of the wood samples were also tested. The results showed that silica sols could effectively permeate and fill into the sample’s pore structure. Bending properties of the samples modified with acidic silica sols, increased more than those modified with alkaline silica sols. Thermal degradation residual weights increased with decreased carbonization temperature and increased apparent activation energy, which exhibited improved flame retardance.