The weight reduction and dyeing properties of circular-knitted fabric manufactured from sea-island-type polyethylene terephthalate (PET) ultramicrofiber were investigated. The diameter of a single fiber was approximately 600 nm. The results of scanning electron microscopy (SEM) analysis, alkali dissolution ratio measurements, and cationic dye staining method revealed that the dissolution of the sea component from each ultramicrofiber was completed through alkali treatment with 1% NaOH aqueous solution for 30-40 min at $95^{circ}C$. The color yield of the disperse dye on the fabric was observed to be dependent on the dyeing temperature. The color yield increased with an increase in the dyeing temperature up to $120^{circ}C$, and then decreased at $130^{circ}C$. The build-up property was generally good. The wash fastness was poor and the light fastness was very poor.