Polyester ultramicrofiber (UMF) (fiber fineness: 0.05d) and regular fiber (R-PET) (fiber fineness : 2.05d) were annealed at various temperatures ranging from 12$0^{circ}C$ to 22$0^{circ}C$ under tension and tension free for 10 minutes. Weight loss by alkali hydrolysis, dyeability of fibers with disperse dye (C.I. Disperse Red 60), and change of fine structure were studied. Equilibrium dye uptake of R-PET initially decreased with increasing heat setting temperature up to 160~18$0^{circ}C$ and then increased at higher temperature, whereas that of UMF under tension was continuously decreased. The dyeing rate of UMF annealed under tension above 20$0^{circ}C$ remarkably decreased because of changes in the fiber orientation. The intensities of X-ray diffraction peaks of both annealed fibers increased with increasing annealing temperature. The heat setting of UMF under tension increased smoothly the crystalline orientation factor ($f_{c}$), whereas decreased $f_{c}$ a little over 18$0^{circ}C$ under tension free. Birefringence of the heat set R-PET under tension and tension free increased up to 18$0^{circ}C$ and then decreased above 18$0^{circ}C$, whereas those of UMF annealed under tension increased continuously.