This study aims to investigate the relationship between mechanical brillation, morphological properties, and rheological behavior of cellulosic ber. Three types of cellulosic bers were obtained by adjusting mechanical brillation, namely squashed cellulose, incompletely nano brillated cellulose, and completely nano brillated cellulose, respectively. The squashed cellulose with large size and small aspect ratio had low entanglement capacity, thus forming a weak ber network. The corresponding suspension exhibited low viscosity, weak elastic behavior, small yield stress, and low dynamic stability. An obviously increasing aspect ratio and entanglement capacity were observed with increasing mechanical brillation, resulting in entangled ber network structure. Hence, the cellulosic ber suspension obtained by more mechanical brillation exhibited higher viscosity, stronger gel-like behavior, and bigger yield stress. Moreover, the extremely entangled ber network structure has better anti-deformation capacity and recovery capacity.We revealed the fundamental insights into the relationship between morphologies and rheological properties of cellulosic ber, paving the way for designing cellulose-based materials.