The effect of cold work and sensitization treatment on the microstructure, mechanical property and stress corrosion cracking of 304L stainless steels in an 80°C 40 wt% MgCl2 solution was investigated in this research. The results showed that cold work caused obvious work hardening in the 304L stainless steel plate along with a great increase in defects, and it also enhanced strain-induced martensite in the steel. Increased trapping sites for hydrogen due to the defects resulted in a much lower NTS loss of the cold-rolled specimen (11%) than that of the solution-treated specimen (25%), tested in the MgCl2 solution. For the cold-worked and sensitized specimen, the strain-induced martensite formed near the grain boundary provided a short path for hydrogen diffusion and led to a larger NTS loss of 27%. In addition, the fracture morphologies of the cold worked specimen altered from quasi-cleavage to intergranular fracture.