Irradiation creep of modified 316 (PNC316) and 15Cr-20Ni base austenitic stainless steels was determined using the diametral change of the fuel pins, which were irradiated under the fission gas pressurized condition in FFTF to a peak displacement damage of 107 dpa in PNC316 and 120 dpa in 15Cr-20Ni base. The creep compliance B⁰ and creep-swelling coupling coefficient D were found to be 1.1 to 3.0 × 10^-6 MPa^-1 dpa^-1 and up to 1.3 × 10^-2 MPa^-1 for instantaneous volumetric swelling rate, respectively. Those are almost the same level as those derived using the pressurized tubes irradiated in FFTF-MOTA. D values tend to decrease and disappear with developing void swelling.