The weight reduction and dyeing properties of circular-knitted fabrics, fabricated from sea-island-type polyethylene terephthalate (PET) ultramicrofiber with a single fiber that is approximately 400 nm in diameter, were investigated. It was found that the dissolution ratio was dependent on the process temperature and the concentration of the sodium hydroxide (NaOH) solution used during alkali treatment. The use of a penetrating agent reduced the dissolution rate when using a 1 wt% NaOH solution, but accelerated the dissolution when using a 2 wt% NaOH solution. The dissolution ratio reached its theoretical maximum when the fabric was treated at 95 ℃ for approximately 40-50 min with a 1 wt% NaOH solution and for approximately 15-20 min with a 2 wt% NaOH solution. Under these treatment conditions, a large number of the sea components were dissolved, although some island components were not separated even in the case of treatment at 95 ℃ for 50 min with a 1 wt% NaOH solution, as monitored by scanning electron microscopy (SEM). The use of a cationic dye staining method also allowed for observation of the dissolution behavior. The color yield of the disperse dye on the alkali-treated fabric was dependent on the dyeing temperature, and the K/S value decreased as the dyeing temperature increased from 110-130 ℃. The build-up property of the fabric was generally good, the wash fastness was very poor to poor, and the light fastness was poor to fair.