A study on disperse azo black textile dye was attempted. The black dye was obtained as a mixture of disperse azo blue 79, red 1 and yellow 3 dyestuffs. The red 1 and yellow 3 dyes were prepared in laboratory. The disperse black dye was applied to polyester fabrics. Its colour was assessed by tristimulus colorimetry. Light, washing and rubbing fastnesses were determined. Optimal hue was obtained with a mixture of 65% blue, 18% red and 17% yellow dyestuffs by weight. The high values observed for fastnesses were explained by modifying of synthesis techniques. Results of the study revealed that the disperse black dye is a physical mixture of blue, red and yellow dyestuffs and, physical forces are effective on dyeing of polyester fibers by it.