Microstructural features and their related mechanical property changes in the 309L cladding and the heat affected zone (HAZ) of SA508 cl.3 steel were investigated through the use of TEM, tensile and small punch (SP) tests. The specimens were irradiated at 563 K up to the neutron fluences of $5.79{ imes}10^{19}n/cm^2$ (>1MeV). The microstructure of the clad was mainly composed of a fcc ${gamma}-phase$, a low percentage of bcc ${delta}-ferrite$, and a brittle ${sigma}-phase$. Along the weld fusion line there formed a heavy carbide precipitation with a width of $20{sim}40{mu}m$, showing preferential cracking during plastic deformation. The yield stress and ductile-to-brittle transition temperature (DBTT) of the irradiated clads increased. The origin of the hardening and the shift of the DBTT are discussed in terms of the irradiation-produced defect clusters of a fine size and brittle ${sigma}-phase$.