Wide range of fineness of ultra-fine denier PET filaments (0.2~0.01 denier per filament) were prepared and the effects of fineness on the dyeing properties of a disperse dye (CI Disperse Blue 56) were studied. The equilibrium study of the disperse dye on the fibers at various concentrations showed that the isothermal absorption was Nernst type and that the equilibrium adsorption of the dye increased with the increase in fiber fineness. Dyeing rate of the dye increased with the decrease in fiber fineness. The apparent diffusion coefficient of the dye in the ultra-fine denier fibers decreased when the fiber fineness increased because of the limited dye liquor turbulence through the compactly assembled filaments in the ultra-fine denier yarn. The temperature dependence of dye diffusion decreased with the fiber fineness.