The molecular characteristics and the chain stiffness were investigated for fractionated samples of cellulose diacetate (CDA, degree of substitution DS = 2.40) in N, N-dimethylacetamide (DMAc) through the partial specific volume, the viscometric, the sedimentation velocity, and the sedimentation equilibrium measurements at 30 degrees C. It was found that CDA dispersed molecularly in DMAc under the external field such as in the viscometric and the sedimentation experiments. The molecular weight dependence of the intrinsic viscosity [eta] and the sedimentation coefficient at infinite dilution s(0) of the single CDA molecule were expressed by the relation [eta] = 1.10x10(-2) M-w(0.85) (cm(3) g(-1)) and s(0)=2.25x10(-14)M(w)(0.20) (s), which exhibit the stiff, or semiflexible chain nature. The semiflexible chain parameters were evaluated by the Yamakawa-Fujii (YF) theory of the unperturbed wormlike cylinder model, first via a combination of the viscosity and the partial specific volume data (method A) and second via a combination of the sedimentation and the partial specific volume data (method B). Method A gave the chain parameters that q = 8.0 nm, M-L = 523 nm(-1), and d = 0.89 nm, whereas method B gave q = 48 nm, M-L = 560 nm(-1), and d = 0.93 nm. Here q is the persistence length, M-L is the molecular weight per unit contour length, and d is the chain diameter of the wormlike cylinder model. Methods A and B deduce, independent of the method, the definite M-L and d values, which are very consistent with the ordinary reported values. However, q estimated by the two methods differs by about six times the other. This fact suggests that the CDA molecule in the ultracentrifugal field has a conformation different from that in the viscometric shear field : The CDA chain may be highly stiff in the ultracentrifuge because of the situation that the adjacent glucose residues are stuck in a rigid conformation by the double stapled hydrogen bonds between the intramolecular hydroxyls and oxygens.