To constrain the behavior of K isotopes during granitic magmatic differentiation, we present high-precision K isotope data for bulk granitoids and their K-bearing minerals from Dabie and Himalayan orogens in China. Plagioclase displays extremely large K isotopic variation, with δ41K ranging from −0.56 to 1.91‰. Other minerals also have heterogeneous K isotopic compositions, with δ41K varying from −0.56 to −0.24‰ in hornblende, from −0.72 to −0.39‰ in biotite, from −0.79 to −0.47‰ in K-feldspar, and from −0.74 to −0.61‰ in muscovite. The general trend of 41K enrichment follows the order of plagioclase ≫ hornblende > biotite ≈ K-feldspar ≥ muscovite, which can be attributed to the difference in KO bond strengths. Both δ41K of plagioclase and K isotope fractionation factors between plagioclase and other minerals are negatively correlated with K concentration in plagioclase, suggesting a compositional control on isotope fractionations. Inter-mineral K isotope fractionations among other minerals are roughly temperature-dependent. Potassium isotopic compositions of whole rocks display measurable variations (δ41K = –0.69 to −0.28‰), which are correlated with proxies for plagioclase fractionation (e.g., Sr and Rb/Sr). The correlation indicates that fractional crystallization may account for the whole-rock K isotopic variations observed in high-silica igneous rocks, which is supported by modeling of fractional crystallization using the apparent isotope fractionation factors determined in this work. Our study demonstrates the existence of large K isotope fractionation during granitic magmatism, making K isotopes a potential tool for studying differentiation of felsic magmas.