A 304L stainless steel water degrader was irradiated with 800 MeV protons at Los Alamos Neutron Science Centre (LANSCE) up to 8.5 dpa at temperatures up to 250 degreesC. Tensile tests showed that hardening and embrittlement were induced in the material. In order to understand the irradiation hardening and embrittlement mechanism, the microstructure in both as-irradiated and deformed material has been studied. The results of TEM investigations show that in the as-irradiated material the main features are: (a) very dense small defect clusters, part of them can be resolved as stacking fault tetrahedra (SFT), with a mean size of about 1.6 run independent of irradiation dose; (b) large Frank loops. whose size increases with dose but whose density varies little with dose; (c) amorphization of precipitates; and (d) no observable helium bubbles or cavities. The main feature in the deformed material is the formation of twin lamellae and bundles of twin lamellae. In all of the four samples (0, 0.7, 3.4 and 6.8 dpa) studied, dense twin lamellae have been observed. The twin planes are {111}. Similar to channels observed in irradiated and deformed fcc pure metals, the original microstructures inside the twin lamellae, namely small clusters and Frank loops, have been removed. The width of the twin lamellae and their bundles varies from a few nanometers to more than 100 nm. The structure outside the twin lamellae is little changed. (C) 2001 Elsevier Science B.V. All rights reserved.