Initiation and propagation of IGSCC for sensitized Type 304 stainless steel in dilute sulfate solutions have been investigated by the slow strain rate test (SSRT) with a CCD camera system. An increase in concentration of sulfate ion was found to cause decrease in crack initiation strain and fracture strain, resulting in the increase in the IGSCC susceptibility. Statistical analysis made it clear that the increase in concentration of sulfate ion had little effect on crack initiation frequency, and crack initiation time obeyed the exponential probability distribution, but the crack velocity obeyed the Weibull probability distribution. With increase in the concentration of sulfate ion, the crack velocity increased, and the number of cracks decreased. On the contrary, the mean crack velocity decreased and the number of cracks increased with time. The dependence of the crack velocity on the concentration of sulfate ion and the number of active cracks under potentiostatic SSRT conditions was successfully explained by a multiple cracks model combined with the mass transport model.