Abstract: Abstract: In this study, cellulose nano-fibril (CNF)/nano-silicon dioxide composites were prepared by mimicking the fast paper-making method. Mechanical property, thermal degradation and moisture adsorption of the composites were studied. The results revealed that tensile strength and modulus decreased as the loading of nano-silicon dioxide increased. The measured density value of CNF was 1 305.04 kg/cm3 while the density of CNF with 20% nano-silicon was only 1 132.9 kg/cm3. Tensile strength of CNF was 120.98 MPa and Modulus was 6.41 GPa. As the loading of nano-silicon dioxide increased, the tensile strength and modulus decreased sharply. When the content of nano-silicon dioxide reached 20%, the tensile of nano-composite was 49.41 MPa and the modulus was 2.96 GPa. The thermal stability of nano-composite was improved after adding nano-silicon dioxide. The onset cellulose decomposition temperature was around 270°C and weight loss in this period was around 4%. Although the onset decomposition temperature of nanocomposites did not increase, the lower weight loss indicated less initial decompositions in this stage. In addition, as the content of silica became higher, the char residue increased. The amount of dry plain CNF was 23%, noticeably smaller than that of corresponding CNF/ nano-silicon dioxide samples which ranged from 30% and 65%. Degradation models including the Kissinger, modified Coats-Redfern and Flynn-Wall-Ozawa (F-W-O) methods were utilized to calculate the activation energy. The Kissinger method led to an apparent activation energy ranging from 150 to 225 for all films. NFC with inorganic silica normally showed a higher activation energy than the control, high clay content also resulted in high activation energy. Results from the modified Coats-Redfern and F-W-O methods were similar (activation energy ranged from 180 to 220) to the observations. Nano-silicon dioxide provides barrier to the oxygen which leads to an improvement in flame retardant property. Limiting oxygen index of the tested CNF was 21.8 which is similar to normal paper but lower than CNF with nano-silicon dioxide. Limiting oxygen index of the CNF with 20% nano-silicon dioxide was 24.20%. The CNF/nano-silica dioxide composites were placed in a container with relative humidity 95% for 24 hours and moisture content of composites increased as the time continued. Among all composites, CNF/nano-silicon dioxide showed more moisture sensitivity than plaint CNF. The higher the nano-silica dioxide content was, the more moisture was absorbed. The final moisture content (FMC) of CNF was around 20% while the FMC of CNF with 20% nano-silica dioxide was around 26%. Contact angle was used to determine surface wettability of these nanocomposites. The contact angle of plain CNF was 55.9°. As the nano-silicon dioxide content increased, the contact angle of nanocomposites decreased. When the loading of nano-silicon dioxide was lower than 10%, the contact angle of nanocomposite was similar with CNF. However, as the loading of nano-silicon dioxide increased to 15%, the contact angle of the nanocomposite was only 45.4°. When the content of nano-silicon dioxide was 20%, the contact angle decrease to 40.4°. In conclusion, mechanic and water adsorption properties of cellulose nano-fibril were affected by added nano-silicon dioxide amount, but the thermos property of the cellulose nano-fibril/ nano-silicon dioxide was stable.