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Irradiation assisted stress corrosion cracking (IASCC) behavior and irradiated microstructural features of cold-worked 316 stainless steel flux thimble tube specimens were investigated in 340 °C simulated PWR primary water. It covers three dose levels (∼40, ∼60 and ∼100 dpa) and five stress levels (35%, 40%, 50%, 60% and 70% of irradiated yield strength at high temperature. Crack initiation in highly irradiated materials occurred rapidly in 33 failed specimens at high-stress level. A clear dose dependence for IASCC initiation was observed. The susceptibility to IASCC initiation reached saturation around 40–60 dpa. Fracture surface of different dose level O-ring specimens showed that intergranular cracking was the main fracture mode. Discrete slip steps were observed on intergranular fractured regions of each dose level specimens, which is associated as evidence of dislocation channeling. A threshold stress level which was close to 30% yield strength was determined on various testing results from literature and this study. Data scatter was observed for those results from different studies but all agreed on that the failure rate increases with stress level and neutron dose.