: Polyolefins have significantly obtained a main position in packaging industry because of their low cost, light weight, required properties, and low-energy consumption during their processing. To overcome this problem, production of degradable and biodegradable polyethylene is necessary. Starch is a widely used polymer in thermoplastic applications because of its biodegradability. Cellulose is the most available biopolymer on earth occurring in wood and other lignocellulosic sources which can be isolated as nanofibers to reinforce composites. Cellulose nanofibers were isolated from wheat straw by a chemical-mechanical procedure. Initially, wheat straw fibers were subjected to a chemical process to eliminate lignin, hemicelluloses, and pectin. Wheat straw was soaked in NaOH solution for pretreatment to swell cell walls enabling chemical molecules penetrate through the crystalline region of cellulose and also enhance dewaxing. Hemicellulose and pectin were removed from pulp in acid hydrolysis stage. Since the acid hydrolysis is a critical step in nanofiber extraction, the HCl solution concentration and hydrolysis time were optimized based on the highest degree of crystallinity using statistical response surface methodology (RSM). Afterward, soluble lignin and klasson lignin (insoluble lignin) were eliminated in alkali treatment by dilute NaOH solution and in bleaching stage by sodium chlorite (NaClO 2 ) solution, respectively. The chemical composition of fibers at different stages of chemical treatment were analyzed and showed an increase in ?-cellulose content. In the second part of the treatment, obtained chemical-purified cellulose microfibers were mechanically disintegrated into nanofibers employing a sonicator at 400 W power. The morphology and size distribution of nanofibers were characterized using scanning electron microscope (SEM). The average diameter and aspect ratio of nanofibers were obtained to be 45 nm and about 23-54, respectively. In addition, the thermal properties of nanofibers and untreated fibers were studied by Thermal gravimetric analyzer and found to be improved dramatically. The degradation temperature of nanofibers reached beyond 300 °C. These mentioned properties are highly suitable for using cellulosic nanofibers in polymeric nanocomposites. In this research, the effect of bio nanofibers on physical and mechanical properties of polyethylene/starch/polyethylene grafted maleic anhydride blend was investigated. Nanocomposites reinforced with different amounts of CNF from 6 to 14% were prepared using an internal mixer followed by a single screw extruder. Flow properties of nanocomposite blend were determined by Melt Flow Index (MFI) and viscosity measurement. MFI was decreased and viscosity increased. To evaluate the mechanical performance of nanocomposites, tensile properties were measured. It was observed that by increasing CNF content tensile strength and elongation at break were decreased, whereas Young’s modulus was increased. In addition, water absorption and degradability under soil was increased with adding CNF in nanocomposites which can enhance degradation of the composite. Finally, it was demonstrated that nanocomposite containing 9% CNF has desirable flow, mechanical, degradability, and water absorption properties. Key words: Wheat straw, Cellulose nanofibers, Total crystallinity index, optimizing, nanocomposite, degradability.