We report here in situ measurements of Rb and Sr isotopes in K-rich minerals (muscovite, phlogopite, and K-feldspar) using Sensitive High-Resolution Ion MicroProbe (SHRIMP). Several grains of the K-rich minerals were extracted from eight granitic rocks with various formation ages and mounted on epoxy-resin disks. Chemical compositions were measured by an electron probe microanalyzer (EPMA). Then they were set in the source chamber of SHRIMP. A 4-5 nA mass filtered O₂^- primary beam was used to sputter a 30-μm-diameter flat-bottomed crater and secondary positive ions were extracted by a 10 kV accelerating voltage for mass analysis. A mass resolution of 8900-9300 at 1% peak height was attained to reduce isobaric hydride interferences such as ⁸⁷RbH⁺ on ⁸⁸Sr⁺. The magnet was cyclically peak-stepped from mass 84 (⁸⁴Sr and ²⁸Si⁵⁶Fe) to mass 88 (⁸⁸Sr and ³⁰Si⁵⁸Fe), including the background, mass 85 for ⁸⁵Rb and ²⁹Si⁵⁶Fe, mass 86 for ⁸⁶Sr and ³⁰Si⁵⁶Fe, and mass 87 for ⁸⁷Rb, ⁸⁷Sr, and ²⁹Si⁵⁸Fe, respectively. No SiFe compounds of any mass number contributed significantly for the samples analyzed. Observed ⁸⁷A/⁸⁵A ratios (where A is an arbitrary element) were plotted against ⁸⁸A/⁸⁵A ratios for each sample. A least-square fitting was made in the diagram. The gradient and Y-intercept correspond to initial ⁸⁷Sr/⁸⁸Sr and initial ⁸⁷Rb/⁸⁵Rb ratios or present (⁸⁷Rb/⁸⁵Rb)+(⁸⁷Sr^*/⁸⁵Rb) ratios, respectively, where ^{“ * ”} denotes radiogenic. The initial ⁸⁷Sr/⁸⁸Sr ratios are mostly consistent with the upper mantle ⁸⁷Sr/⁸⁸Sr ratios of various ages within experimental error. There is a positive correlation between the initial ⁸⁷Rb/⁸⁵Rb ratios and formation ages of the granitic rock samples, as it is expected from ⁸⁷Rb decay. Further evaluation of the instrumental isotopic mass fractionation and relative sensitivities for Rb and Sr are required before the in situ ⁸⁷Rb-⁸⁷Sr measurement is routinely applied as a dating method.