Dyeing properties of compounded polyethylene terephthalate fibers with a polyesteramide hyperbranched polymer were investigated in terms of their thermodynamic parameters, including standard affinity -Δμ⁰, enthalpy change ΔH⁰ and entropy change ΔS⁰. Results were then compared with corresponding properties of the virgin PET. The results showed that the isotherms were similar to that of virgin PET following as linear isotherm. The standard affinity of the dye to the compounded samples showed higher value than that of virgin PET. As the content of additive increased, the standard affinity of dye to the PET increased. The bath containing virgin PET displayed the highest negative values of the ΔH⁰ and ΔS⁰, while that of the PET with 2 hyperbranched polymer showed the lowest negative value. The surface structure and thermal behavior of the samples were determined by FTIR and DSC, respectively. DSC tests showed that the crystallinity and glass transition temperature of samples were decreased.