In order to estimate equilibrium chain rigidity, literature data for 12 systems of cellulose and its derivatives and solvents were analyzed on the basis of a wormlike chain model. The persistence length q was calculated by four methods. The q values at the unperturbed state (qBD0) agreed closely with those derived using the pearl necklace model. The q values evaluated by the Yamakawa–Fujii (YF) theory were smaller than those obtained by the Benoit–Doty equation (qBD); this discrepancy was considered due to the neglect of the partially free draining effect in the YF theory. With increasing solvent polarity, the polymer chain becomes stiffer owing to an increase in solvent–polymer interaction. Most of qBD0 for cellulose derivatives were in the range between 3×10−7 and 8×10−7 cm, except that for cellulose nitrate. Cellulose and its derivatives may be considered semiflexible.