Nylon 6 ultramicrofiber(UMF, monodenier 0.07d) and regular staple fiber(monodenier 2.05d) were dyed with acid and disperse dyes to investigate the effect of the difference of the fiber fineness. Also X-ray diffraction pattern, birefringence, DSC thermogram, moisture regain and water absorption of these fibers are measured. The dyeing rate of nylon UMF with acid dyes is increased compared with that of regular fiber, but not increased for disperse dyes. Also the saturation dye uptake of UMF with acid dyes is higher than that of regular fiber, while it is unchanged for disperse dyes. The moisture regain of UMF is similar to the regular fiber, whereas the water absorption of UMF is two times than that of the regular fiber. The crystallinity percentage of UMF is higher than that of regular fiber.