Cellulose aerogels are green, ultralight and highly porous materials which are promising alternatives to common thermal insulation materials. In this study, cellulose aerogels were prepared from rice straw via a sol-gel process with aqueous solutions of NaOH-polyethylene glycol (PEG) and freeze drying method. The materials were then functionalized with methyltrimethoxysilane (MTMS) to achieve hydrophobicity. The morphology, pore structure and physical properties of the aerogels were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), thermal conductivity, nitrogen adsorption and water contact angle measurements. The results demonstrated that the cellulose aerogels possessed ultralow densities, high porosities, superhydrophobicity and very low thermal conductivities.