A detailed observation on the penetration profile of chloride ions through a crack in concrete structures was carried out. Cracks were induced using a splitting tensile test, and average crack width was measured using an image analysis technique. Chemical analysis tests were conducted on cracked specimens, which were exposed to NaCl solution at a temperature of 20°C, after being cured for 28d. The results showed that the concrete specimens having cracks showed rapid penetration of chloride ion. The diffusion coefficient calculated according to Fick’s law also indicated that the chloride permeability increased with the increasing crack width.