We have investigated the wavelength dependence of the birefringence (Delta n) for cellulose acetate propionate (CAP) films stretched at various draw ratios (DRs) and strain rates (SRs) by comparing with the result of cellulose triacetate (CTA). CAP exhibits extraordinary wavelength dispersion of An although CTA shows ordinary dispersion, indicating that An of CAP is determined by the acetyl and propionyl groups. For CAP, the extraordinary dispersion becomes stronger at higher DR and SR. Thermal analysis suggests that hot stretching induces crystallization of CAP and the crystal size increases with increasing DR and SR. Furthermore, the two-dimensional X-ray diffraction patterns of CAP and CTA (a semicrystalline polymer) exhibit orientation of the induced crystal. These results mean that the acetyl orientation in CAP becomes stronger than the propionyl orientation. This conclusion is reasonable because the acetyl group is more tightly confined to the pyranose ring than the propionyl group. (C) 2017 Elsevier Ltd. All rights reserved.