To investigate the effect of carrier on the dyeability of poly(ethylen terephthalate)(PET) fiber, equilibrium dye uptake and initial dyeing temperature were measured and discussed in connection with solubility parameter. It was shown that the equilibrium dye uptake for carrier-added dyeing increased with smaller component solubility parameter difference between PET and carrier, and lower initial dyeing temperature. For carriers with similar solubility parameter and molar volume, the one which has phenyl ring and carbonyl group in its molecular structure exhibited greater carrier activity. In this experiment, the most effective carrier for PET fiber was benzaldehyde and acetophenone, which can interact with PET fiber most effectively by overlap of p-orbitals.