The effects of noncircular cross-section (NCCS) poly ethylene terephthalate (PET) filaments and its shape factor on deep-black-coloring of dyed fabrics were investigated by comparing to that of the circular cross-section PET ones. Indexes such as K/S, L* and Integ values were used for characterizing the deep-black-coloring effect on fabrics. The results indicated that fabrics made with NCCS PET filaments exhibited good deep-black-coloring effects. The calculated shape factor of the NCCS PET fiber had a significant correlation with the degree of deep-black-coloring exhibited by the fabric made from the fibers. A qualitative optical analysis of the NCCS PET fibers was carried out to explain the causes of the deep-coloring of the NCCS fibers. This analysis implies that the contours of the NCCS fiber composed of surfaces with varied curvature increase the scattering of light by lowering specular reflection and increasing interior reflected and refracted light. This, in turn, strengthens the deep-coloring effect. (C) 2013 Wiley Periodicals, Inc.