The present study aims to investigate the structure-morphology-rheology relationships for cellulose nanoparticles (CNPs), including cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs). CNCs were extracted from never dried CNFs using sulfuric acid with controlled hydrolysis time. The crystalline structure, surface charge, morphology, and rheological behavior of the CNPs were measured and contrasted. The CNF suspensions exhibited rigid solid-like viscoelastic behavior even at a low concentration due to the formation of a highly entangled network. Upon acid hydrolysis, the network of rigid, long, and highly entangled nanofibers was eliminated, resulting in a significant loss of viscoelastic properties. Both steady-state and dynamic rheological measurements showed that the rheological behavior of the CNC suspensions was strongly dependent on the concentration and acid hydrolysis time. The CNC suspensions exhibited elastic gel-like rheological behavior at high concentrations but viscous liquid-like rheological behavior at low concentrations. Longer acid hydrolysis time produced CNCs with a lower aspect ratio, leading to higher critical transition concentration for the formation of anisotropic phase. The aspect ratio of CNCs was predicted from the intrinsic viscosity using the Simha's equation. The theoretically predicted aspect ratio values corresponded well with the transmission electron microscopy results. Finally, the network of CNF and CNC suspensions were schematically proposed. (Graph Presented). ©2015 American Chemical Society.