The behaviour of stress corrosion (SC) crack initiation and propagation of type 304 stainless steel in 35wt.% $MgCl_2$ has been studied as a function of temperature and applied stress at the applied potential of -$106mV_{SHE}$ using electrical potential method under constant load. In the present work, the incubation and progagation periods for SC crack have been differentiated by the electrical potential method. The optical microscopy shows that the pit is a primary source of SC crack initiation under 212MPa (0.90YS). SC crack initiation stage is composed of three steps ; 1) pit initiation 2) Pit growth to critical size 3) Crack propagation in the lateral and through - thickness directions. The fractograpy of SC crack propagation shows the transition of fracture mode from fully transgranular mode to the mixed zone with intergranular mode in the interior of the specimen. The intergranular portion is nearly independent of temperature and increases with the applied stress. The activation energy of electrochemical reaction and the stress contribution for SC crack propagation under 212MPa are evaluated to be 63KJ/mole and 8KJ/mole, respectively. It is suggested that the former is for ion diffusion, anodic dissolution and repassivation reaction, the latter is for the destruction of the passivated film. The average SC crack propagation rate at 124℃ and - $106mV_{SHE}$ under 212MPa is calculated to be 1.24×$10^{-4}$mm/sec.
