This thesis reports on the synthesis of a hydrogel made from oxidized tannic acid (OTA) nanoparticles and TEMPO-oxidized cellulose nanofibers ((TOCN). We prepared the OTA nanoparticles by oxidizing tannic acid (TA) under slightly alkaline conditions. Fourier-transform infrared spectroscopy (FTIR) and Thermogravimetric Analysis (TGA) were used to probe the chemical and structural changes of the OTA nanoparticles during the oxidation. The morphology of OTA particles was observed using a Scanning Electron Microscopy (SEM). OTA nanoparticles were added into a TOCN suspension to form TOCN/OTA hydrogels at 60 ℃ for 28 hours. In addition to hydrogels, TOCN/OTA aerogels were also prepared through freeze-drying. We noted that the hydroxyl groups on the surface of TOCN and OTA could form intra- and interchain hydrogen bonds, while the flexible cellulose nanofibers could form high physical entanglements. TGA spectra verified the improved thermal stability of the cellulose aerogel when OTA nanoparticles were incorporated. We also investigated the effect of the weight ratio of TOCN/OTA on the thermal and viscoelastic properties of the hydrogels and aerogels. Samples prepared at higher TOCN/OTA weight ratios exhibited higher thermal stability. Rheological tests indicate that TOCN/OTA hydrogels act as elastic solids under cyclical deformation. An optimal weight ratio of TOCN/OTA was determined. TOCN/OTA hydrogels were prepared from natural resources, and through ‘green’ methods, and it is expected that these new materials could find applications in medical and hygienic products as well as in environmental remediation processes.