A glass fiber reinforced plastic (GFRP) with cyanate ester resin was fabricated and neutron irradiation tests up to 1×1022 n/m 2 of fast neutron with over 0.1 MeV energy were carried out in fission reactor. The fabrication process of cyanate ester GFRP was established and a collaboration network to perform investigations on irradiation effect of superconducting magnet materials was constructed. Three kinds of samples were fabricated. The first was CTD403 GFRP made by NIFS, the second was (cyanate ester + epoxy) GFRP provided by Toshiba, and the last was CTD403 GFRP made by Toshiba. The irradiation was carried out at JRR-3 in Japan Atomic Energy Agency using Rabbit capsules. After the irradiation, short beam tests were conducted at room temperature and 77 K and interlaminar shear strength (ILSS) was evaluated. The irradiation of 1×1021 n/m2increased ILSS a little but 1×1022 n/m2 irradiation decreased ILSS to around 50 MPa. These tendencies were observed in all three kinds of GFRPs.