Two kinds of sea-island type polyester ultramicro fibers (fiber fineness : 0.01 and 0.05 denier) were treated with NaOH varying time and concentration. Surface morphology of the treated fibers with alkaline weight loss was observed by SEM. The treated effects were investigated by measuring density, melting temperature, and X-ray diffraction patterns. The surface morphology of the polyester ultramicro fiber was changed by NaOH concentration. Weight loss of 0.01d fiber was much larger than that of 0.05d fiber. Density and crystallinity were increased with weight loss of fiber. After the weight loss had finished, the density and crystallinity were decreased because of attack of island partition of the fiber. A melting temperature$(T_m)$ is $250^{circ}$ at untreated fiber on the whole and in 0.05d fiber the $(T_m)$ is $252^{circ}$ at untreated. In 0.01d fiber the $(T_m)$ was increased with weight loss of fiber.