Stainlesssteel 304 is being widely used in water supply system, and the dissolution of Cr⁶⁺ from materials of the system into tap water is one of the critical indexes of safety assessment. In the present work, different samples of stainless steel 304 components such as pipe, bar, plate, casting, and powder metallurgical product, were immersed in simulated tap water for 22 d, then the concentration of the dissolved Cr⁶⁺ in the water was determined by means of colorimetric of diphenylcarbazide with a spectrophotometer V-1800PC, while the microstructure of the samples before and after test was carefully examined by means of metallographic microscope MEF4A with image analysis system Q500 and SEM interms of morphology of grains, phase composition and distribution of inclusions etc. The results showed that the amount of dissolved Cr⁶⁺ versus time were quite different for sample to samples. The larger diameter and quantity of spherical inclusions in steels would be much favorable to the dissolution of Cr⁶⁺ for the same immersion period. In general, the spherical inclusions were complex oxides rich in Cr and Mn. In order to reveal the effect of MnO₂ on the dissolution of Cr⁶⁺ from the steel, artificial inclusions of Cr₂O₃ and Cr₂O₃+MnO₂ were introduced respectively into the steel by powder metallurgy, therewith the immersion test results showed that the dissolution of Cr⁶⁺ increased with the increasing amount of Cr₂O₃ and MnO₂. These results confirm that the inclusions of Cr/Mn complex oxides play important role in the dissolution process of Cr⁶⁺, i.e. they are not only the source of Cr³⁺, but act also as oxidant to transform Cr³⁺ to Cr⁶⁺ in the Cl^- contained solution. It can be inspired from these results that the Cr⁶⁺ dissolution could be significantly decreased by controlling the density and size of Cr/Mn oxides inclusions.