The first trial irradiation tests of pressure vessel steels scheduled by the Steel Irradiation Joint Committee of Japan (SIJC) were begun in 1963 and completed in 1967. ASTM Specification A302 Grade B, modified A302B with nickel, HY-80, and several other domestic-made low-alloy high strength steels were selected. Specimens were prepared from heavy thick plates up to 250 mm thickness, and experimental plates up to 50 mm thickness, both of which contained weld zones. Specimens from ASTM Committee E-10 were adopted as a reference material. Througout the test sequence, other objectives were to clarify the preliminary effects on irradiation of (a) steel melting processes, for example, vacuum degassing, basic oxygen (L-D) converter smelting, or aluminumnitride treating; (b) thickness of plates and microstructures as affected by heat treatment; (c) welding and stress relieving; and (d) specimen size. All specimens were encapsulated in newly designed helium-tight aluminum sheaths. A fusible metal gap was used to control high-temperature irradiation, and to ensure uniformity of neutron flux a rotating retractable rig device was adopted. The irradiation tests were carried out in the BR-2 reactor of the Centre d'#x00E9;tude de l'#x00E9;nergie Nucléaire (CEN) in Belgium, and the postirradiation tests were carried out in the center's radiation laboratory. In almost all cases fast neutron fluxes ranged from 3 to 4 x 1019 n/cm2 (over 1 MeV), and irradiation temperatures were 75 and 260 C. Postirradiation tests included tension, impact, hardness, and microstructure examinations, but evaluation of the results were based mainly on the transition curves derived from standard Charpy impact tests. Significant results were as follows: (1) The effects of smelting practice are noticeable; the L-D process and the aluminum-nitride treatment, in particular, show good effects on the reduction of irradiation sensitivity. (2) The irradiation sensitivity of thick plates is largely influenced by thickness, that is, mass effect—and heat treatment. (3) In general, weld zones are somewhat more sensitive than the parent metal. (4) The shift of transition temperature (ATr) of the ASTM reference steel falls into the range of the trend band established by studies in the United States, but most specimens show much less sensitivity than the reference steel as measured by ATr except for long-time heated experimental specimens.