The holding time during solution heat treatment of unstabilized austenitic stainless steels as specified in the nuclear regulatory requirements was investigated. The sensitized 2.54cm thick specimens held at 675℃ for 1 h were rejected by ASTM A262 test, due to the large amount of chromium carbide precipitated in the form of 50~300nm particles at the grain boundaries. They also showed about 10.8% of DOS in the DL-EPR test. However, solution heat treatment of the sensitized specimens at 1,038℃ and 1,121℃ for at least 1 min resulted in the complete dissolution of chromium carbide into the grains, and they passed ASTM A262 test and showed less than 0.01% of DOS in the DL-EPR test. As a result of solution heat treatment at 1,038℃ for 5 h of the 25.4cm thick specimen sensitized at 675℃ for 10 h, it passed ASTM A262 and DL-EPR test at any position in the specimen thickness. While the specimen surface showed a step structure without the precipitation of chromium carbide and a DOS less than 0.01%, towards the center, a dual structure was observed. It exhibited about 0.6% of DOS due to the longer exposure time to the sensitization range of 427~816℃. Considering the minimum time in which the chromium carbide precipitated at the grain boundary at 1,038℃ was completely dissolved into the grain, and the maximum delay time for the center of the specimen to reach 1,038℃ rather than the surface, the holding time for complete solution heat treatment to the center was found to be up to 2 min per 2.54cm of material thickness. The solution heat treatment for 0.5~1.0 h per 2.54cm of material thickness at 1,038~1,121℃, which is employed in the nuclear power industry, was proven to prevent grain boundary corrosion by inhibiting the sensitization of unstabilized austenitic stainless steels.