Cellulose nanofibrils (CNFs) and cellulose nano crystals (CNCs) with high and low aspect ratios, respectively, were prepared from wood cellulose by catalytic oxidation with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and subsequent sonication in water. Cavitation-induced force was used to prepare TEMPO-CNCs from aqueous suspensions of TEMPO-oxidized celluloses. Aqueous dispersions of the TEMPO-CNF and TEMPO-CNCs with different solid concentrations were prepared by dilution or condensation. Dynamic light scattering (DLS) was used to determine the solid concentrations at the transition points from the dilute to semidilute regions and from the semidilute to dense gel regions in the aqueous TEMPO-CNF and TEMPO-CNC dispersions. All the DLS data corresponded well to the fitting curves of the normalized time-correlation functions. The solid concentration at the gelation point increased from 0.40% w/v for the TEMPO-CNF dispersions to 1.71% w/v for the TEMPO-CNC dispersions, and the aspect ratio decreased from 134 to 57, respectively. The solid concentrations of the TEMPO-CNF and TEMPO-CNC dispersions at the gelation point calculated using effective medium theory based on their aspect ratios corresponded well with those experimentally determined by DLS.