Dynamic viscoelasticity measurements were performed for aqueous dispersions of cellulose nanofibers prepared by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and subsequent mechanical disintegration in water. The frequency dependence of the storage and loss moduli of 0.02% (w/v) dispersions of TEMPO-oxidized cellulose nanofibers in water showed terminal relaxation behavior at relatively lower angular frequencies. This strongly suggests that each cellulose nanofiber in the dispersion behaves as a semiflexible rod-like macromolecular chain or colloidal particle. Furthermore, a clear boundary was observed between the terminal relaxation and rubbery plateau regions. The longest viscoelastic relaxation time, τ, was estimated from the angular frequency, corresponding to the boundary point, and the average length of the cellulose nanofibers, L, was estimated using the equation τ = πη(s)L(3)/[18k(B)T ln(L/d)]. The equation gave a value of L = 2.2 μm, which was in good agreement with TEM observations.