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Highly crystalline samples of cellulose triacetate (CTA) were prepared from cellulose I by heterogeneous acetylation in an equal amount mixture of acetic anhydride and toluene. Highly crystalline and oriented films of cellulose I from green alga Cladophora sp. was used as a starting material. Prepared samples were studied by FT-IR, solid-state 13C NMR, and X-ray diffraction. Spectroscopic data indicate a complete degree of acetylation. In the solid-state 13C NMR spectrum, each carbon atom appears only as a single resonance, indicating that the asymmetric unit in the CTA I crystal includes only a single chemical repeat unit. This behavior is in contrast with the currently accepted crystal structure of CTA I, in which the unit cell includes two parallel polymer chains. In the fiber X-ray diffraction, prepared samples show high crystallinity and good orientation. A new unit cell is proposed, and the crystalline structure was refined using molecular modeling. Possible conformations of the acetate side chains are also considered. The proposed models include a single polymer chain in a 21 helical conformation placed in a monoclinic unit cell with a = 0.5939 nm, b = 1.1431 nm, c = 1.046 nm, and γ = 95.4°. Agreement between observed and predicted diffraction intensities, compared both by visual methods and in terms of the crystallographic reliability index R, is good with R = 0.22.