A heat resistant colorant for mass-coloring polyester such as polyethylene terephthalate, containing 100 parts of pigments and/or dyes and 42.about.2,000 parts of a liquid aliphatic polyester dispersing medium having a viscosity at 25.degree. C. not exceeding 150 poises and a hydroxyl value not exceeding 25 mgKOH/g. The dispersing medium contains a polycondensate of alkylene glycols including at least 30% by weight of 1,3-butanediol with at least one aliphatic dicarboxylic acid, which polycondensate has at least one terminal group blocked with a monohydric alcohol. The colorants are easily blended and uniformly dispersed in a fiber grade polyester, without causing degradation of the polyester. Fibers mass-colored by the colorant are excellent in stability during their manufacture and uniform in quality.