In the recent decade stainless steels have been widely used in the water treatment plants (WTP) the water reservoirs (WR) and the water supplying pipes in South Korea. The corrosivity of those environments was dependent of the content of residual chlorine in the water. This study investigated how severe the corrosion environments of water reservoirs were and how different the corrosion resistance of different stainless steels was in the water reservoir. Five stainless steels of STS304 (18Cr-8Ni) STS316L (18Cr-10N-2Mo) STS444 (19Cr-1Mo) STS329LD (20Cr-3Ni-1.5Mo) and STS329J3L (22Cr-6Ni-4Mo) were exposed in the simulating water reservoir environments for 20 days. Each sample was cut and inserted into epoxy mold where the exposed area of the sample was 5x5mm. Counter electrode and reference electrode of platinum wire were also inserted into this mold which were located next to working electrode. This mold was placed under the roof in the simulating water reservoir facing the test surface downward. Thin water film was formed on the sample surface. For the test period of 20 days potable water containing the residual chlorine of 0.5ppm continuously flew in the simulating water reservoir and the corrosivity in the water film on stainless sample was getting more severe. The pH was mesured lower than 3.0 and the highest content of chloride was around 600 ppm in the water film. In this severe environment typical stainless steel may easily corrode [1]. Cyclic polarization test was carried on each sample after exposure period of 0 1 3 5 7 10 13 and 20 days to evaluate the resistance to the localized corrosion [2]. The sweep rate was 1 mV/sec. To verify the stability of Platinum reference electrode the OCP of Platinum was measured in simulated water reservoir environments against SCE (Saturated Calomel Electrode) for 20 days. The passive current of STS304 increased as the exposure period increased from 0 to 5 days. After 5-days exposure STS 304 showed large area of potential-current hysterisis indicating the evidence of propagation of localized corrosion. STS316L also showed very similar polarization behavior as that of STS304. After 7-days of exposure the passivity of STS316L became unstable showing wide hysterisis.STS329J3L showed very stable passivity even after 20-days exposure with small increase of passive current. The current value was usually lower than 10-6 and the hysterisis was quite small indicating that STS329J3L would not corrode easily in the water reservoir environments. This result was also proved in the field exposure test of STS329J3L in water reservoir for 24 months. In the field exposure test STS304 and STS316L were easily corroded after 3-months exposure. But STS329J3L lasted longer than 18 months without any rusting.[Reference]A. J. Sedriks Corrosion of Stainless Steel 2nd ed. p149-151(1996).K. W. Efird and G. E. Moller Materials Performance July p34 (1979)