As a main structural component of plants, cellulose is naturally abundant and widely distributed on earth. Although the chemical formula of cellulose was determined in 1838, human beings have been using cellulose to produce daily necessities, such as textiles and paper, for thousands of years. In modern society, the applications of cellulose have expanded considerably as the understanding of the chemistry of cellulose has increased, and researchers have been able to alter the structure of cellulose to obtain new properties to meet new requirements. With this strategy of structure alteration, this work demonstrated new methods of hydrophobization of cellulose to prepare emerging materials with new or improved properties.Cellulose is commonly used for reinforcement in composites. In this study, a new type of microcellulose, cellulose oxalate (COX), was used to prepare polymeric composites. COX was first hydrophobized by maleated polypropylene (MAPP) or oleic acid to obtain better compatibility with the hydrophobic matrix of polypropylene (PP). It was found that a composite consisting of 60% PP and 40% MAPP-hydrophobized COX had a 16% higher tensile strength and a 106% higher Young’s modulus than neat PP. When oleic acid was used to hydrophobize COX, it was possible to avoid using toxic catalysts to complete the esterification between COX and oleic acid. This fact makes the process environmentally friendly.Similarly, considering environmental protection, natural compounds were used to prepare hydrophobic cellulosic textiles. Betulin, a kind of wood extract and by-product in the forestry industry, was used to modify the wettability of cellulosic textiles. The textiles underwent a dip-dry-cure process from a betulin solution had a static water contact angle (SWCA) of 153°. Textiles coated by a betulin-based film exhibited a water-repellency score of 80 according to a standard test from the American Association of Textile Chemists and Colorists (AATCC). Betulin-grafted cotton textiles were also prepared, and such textiles showed not only hydrophobic properties (SWCA up to 136°) but also antibacterial properties, with bacterial removal of more than 99%.This thesis proposes that hydrophobized cellulosic materials can be used in various fields, both related and unrelated to waterproof applications. Furthermore, by-products in the forestry industry, such as betulin and oleic acid, are advocated for use because of their biodegradability and value.