We report the viscoelastic relaxation mechanisms of cellulose nanocrystals (CNCs) dispersed individually in fluids. Linear viscoelasticity and flow birefringence of softwood and tunicate CNC/glycerol dispersions in the dilute regime were measured. The obtained results were then compared with molecular theories for the linear viscoelasticity of rigid rods and semiflexible rods at infinite dilution by taking length distributions of CNCs into account. Although CNCs are traditionally regarded as rigid Brownian rods, the viscoelastic relaxation was not explained solely by the rotational motions of rods. Alternatively, molecular theories for semiflexible rods well-described the viscoelastic behavior; the CNCs showed additional relaxation modes derived from microscopic internal motions including "tension" and "curvature", which originated from the finite flexural rigidity. Birefringence relaxation of the CNCs was dominated by the rotational motions of rods. Length distribution functions of the CNCs were thus calculated from the birefringence relaxation and agreed well with their microscopy-determined histograms.