Cellulose nanofibril (CNF) is biodegradable and has high strength, stiffness and Young’s modulus, great capacity of chemical modification and low oxygen permeability. These properties make it a new material useful for a range of application such as barrier for packing, reinforcing material for paper, building blocks, for drug delivery and many others. CNF is usually produced by mechanical treatment, passing a diluted cellulosic suspension through a homogenizer or ultra-grinder. Also, the nanomechanical processing can be combined with chemical or enzymatic pretreatment, aiming to reduce the high energy consumption or to modify the surface of the cellulose for specific applications. The goal of this study is to elucidate the catalytic action of an endoglucanase (EG), under different processing conditions, during the pretreatment of bleached eucalyptus kraft pulp (BEKP) for CNF production using an ultra-grinder. The properties of CNF were correlated with enzymatic hydrolysis condition. So far, time appears to have more influence on fibrils fragmentation since longer reaction time (9h) had similar fragmentation using high and low enzymatic loading.