Abstract: This study describes a two-step alkali/TEMPO-mediated oxidation process to efficiently convert jute fibers into cellulose nanofibrils (CNF) with the same crystalline polymorph and different aspect ratios and concentrations. Varying the concentration of NaClO used in the second oxidative step enables the morphologies of the CNF to be reliably controlled, with fiber lengths decreasing from micrometer to nanometer levels as the amount of NaClO oxidant used was increased. These CNF were used to prepare oil-in-water Pickering emulsions as promising stabilizer, and the effects of aspect ratio and concentration of CNF on emulsifying capacity and emulsion stability were investigated. With the same dosage, it was found that the nanocellulose fiber with the smallest aspect ratio showed the best emulsifying ability and the best emulsifying stability. The results indicated that although the amphiphilic properties of nanocellulose microfibers enabled them to adsorb at the oil-in-water interface and form stable Pickering emulsions, the aspect ratio of microfibers directly influenced the emulsion morphology, and further affected the emulsifying ability and stability. CNF with a larger aspect ratio can connect partial surface of many droplets, forming into a loose multilayer structure; CNF with a smaller aspect ratio is more likely to cover the entire droplet, and due to the mutual repulsion of electrostatic repulsion brought by carboxyl groups contained in CNF, the aggregation between droplets is prevented, and the emulsion droplets are connected into a spatial network structure, thus forming a more stable O/W two-phase structure. In the same aspect ratio, the higher concentration of emulsifying ability is better, as well as the stability. It is important to note that, saturated oil-in-water two phase interface structure cannot accommodate more CNF with the continuous increase of CNF. It can only be spread to the water phase, through local intergranular flocculation or bridging between two phase interface structure, making the viscosity of Pickering emulsions increase until the emulsions lose fluidity. In the range of ensuring the fluidity of emulsion, higher concentration of CNF will make the spatial network structure between droplets more stable, and achieve better emulsifying ability and stability. The results will improve understandings about the mechanism by which CNF emulsifier morphology affects emulsifying capacity and stability, and promote the expansion of its potential applications. Furthermore, nanocellulose with different characteristics can be used as Pickering emulsion emulsifiers to replace solid particles, which is expected to play a great role in the fields of food, medicine, cosmetics and oil recovery.