Oxygen isotope compositions of whole rock and/or mineral separates (quartz and feldspar) have been determined for the granitic and related rocks from the Wolf River Batholith, Wisconsin. Hydrothermal alteration resulting in the decrease of ${Delta}_{Q-F}$/ values was obaserved locally throughout the batholith. Feldspars of different colors (pink, gray and red) were separated whenever feasible and analyzed. Most red feldspars (An$_{10-30}$/) show the highest and constant ${delta}^18O$/ values (9.3~10.0 permil) suggesting nearly complete isotope exchange with hydrothermal fluid. Based on ${delta}^18O$/ values and the alteration temperatures (260~$350^{circ}C$) estimated from fluid inclusion study, ${delta}^18O$/ of fluid is calculated to be $5.0{pm}1.4$ permil. Phanerozoic sedimentary formation water in Wisconsin is most likely the source of the fluid.