The melt viscosity of poly(ethylene terephthalate) (PET) was decreased by the impregnation of an aromatic solvent, such as 1-methyl naphthalene or diphenyl ether, Porous PET granules for impregnation of the solvent were prepared by swollen-state polymerization in hydrogenated terphenyl as the swelling solvent. The PET granules could take up 20% of the solvent by heating with the solvent. On the other hand, PET granules polymerized in the solid-state scarcely absorbed the solvent, except for 1-methyl naphthalene. The apparent melt viscosity of PET decreased to 1/5 by the impregnation of 1-methyl naphthalene (20%). The solvent-containing PET granules allowed extrusion at lower temperature (270 degrees C), which led to a reduction of the degradation of PET and produced high tensile strength (2 GPa) and high modulus (30 GPa) fibres.