An in-beam creep-fatigue testing machine for a miniaturized specimen was developed at National Research Institute for Metals to investigate the synergistic effects of stressing and irradiation damage on the mechanical properties of materials. The loading and displacement measuring system of the machine is outlined in this paper. An in-beam creep-fatigue experiment was carried out with 17 MeV protons for 20% cold-worked 316 stainless steel at 60°C. The specimen size was 4× 10×0.15 mm in gauge with fatigue crack starter notch introduced at the edge. Creep-fatigue (ramp with hold) tests were carried out in stress control (536.4 MPa, 12 s ↔ 236.4 MPa, 1 s; rate = 50 MPa/s). Simple fatigue tests without holding time were also performed in the same condition. Although the lifetime of simple fatigue was substantially prolonged under irradiation, the prolongation was not so much in the creep-fatigue. The holding during irradiation appeared to play some role in reducing the lifetime.