The effects of fiber morphology such as fiber fineness and cross-sectional shape on the dyeing mechanism of polyester filaments were analyzed. Round shaped 50d (1.38 denier per filament or dpf) polyester filament yarn, scalloped oval shaped 75d (2.5 dpf) polyester filament yarn and cruciform shaped 100d (4.7 dpf) polyester filament yarn was studied for this research. The color difference, darkness-lightness variation, color fastness to wash and visual assessment of residual dye bath liquor were investigated. The difference in cross-sectional shape and fiber fineness caused the difference in color and shade, darkness-lightness variation, difference in color fastness to wash as well as residual dye bath concentration. The results showed that 50d filament yarn having round shape showed higher dye uptake (hence lower dye bath residual), lower color difference from the standard and more similarity to standard’s shade in comparison with polyester filament yarns made of 75d (scalloped oval shape cross-section) and 100d (with cruciform shape cross-section).