Weight reduction behavior and dyeing properties of sea-island PET ultra-microfiber knitted fabrics were compared to those on the fabrics of the conventional microfiber. In the dissolution of the sea component out of seaisland type nanofilaments, it is important to determine the optimum alkaline treatment time in order to avoid the further damage of island component. SEM micrographs reveal that the dissolution of the sea component of each nanofilament fabric completed after 30 or 40 min at $95^{circ}C$. Build-up property of disperse dye on sea-island type PET fabric from microfiber or nanofilament was generally good. Color yield of fabric from micro filament was higher than that from the nanofilament. Color yield of the fabric from PET micro- or nano-filament was found out to be dependent upon the dyeing temperature. The color yield decreased as the dyeing temperature increased. Wash fastness was moderate and light fastness was very poor.