In Taiwan, the neutral thermal water of Tatun Volcano Group has seen increased attention in the last several decades, due to the neutral thermal water causes corrosion of pipeline less than acidic thermal water during developing geothermal resource. This research focused on the characteristics of core of neutral thermal water area, and applied the petrography, and X-ray diffraction to interpret water-rock interaction in the neutral thermal water area of Tatun Volcano Group (TVG). In the TVG, the pH value of neutral thermal water is between 5.5 and 6.5, and their stable isotopic values of hydrogen and oxygen range from -24.8 ‰ to -35.5 ‰ and from -6.5 ‰ to -2.5 ‰, respectively. According to the petrography phenomenon, there shown the significant alteration zone between lava and pyroclastic rock, and existed a lot of iron oxide-limonite and montmorillonite. Furthermore, the alteration mineral has iron oxide-limonite and montmorillonite. This study infered to the neutral thermal water was caused by mixing surface water with volcanic gas based on the evidence of iron oxide-limonite. Finally, this study interpreted that the process of the petrography and minerals transformation of andesite by water-rock interaction. First stage, it was fresh rock that is gray andesite. Second stage, it was medium gray andesite with empty space, and calcite has replaced part of plagioclase and groundmass. Third stage, it was dark gray andesite, had cristobalite, with iron oxide-limonite precipitated in empty space, and montmorillonite produced from plagioclase by hydrothermal alteration. Finally, it was brown andesite, had montmorillonite and cristobalite, with a lot of iron oxide-limonite precipitated in empty space and groundmass.

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