Cellulose nanofibrils (CNF) are one of most popular materials in nanotechnologies due to its favorable properties, such as biodegradability and high mechanical performance. However, their nano-/microscopric structure is not fully understood. In this thesis, we studied the structural features of CNF by using atomic force microscopy (AFM) and scanning electron microscopy (SEM), and then assessed the dimensions of single fibers from different wood species. We studied the dependence of cellulose nanopaper strength and toughness on the size of cellulose fibers using dynamic mechanical analysis (DMA). Interestingly, we found that both the strength and toughness increased as the fiber aspect ratio increased. Additionally, stability tests of carbon nanotubes and cellulose nanofibrils (CNT-CNF) solution were conducted by rheological measurement. The solution showed high stability and no visible precipitation. Based on these properties, we fabricated functionalized nanostructured films from CNF and observed promising results from the novel materials.