  We use the method of QCD sum rules to investigate the isospin symmetry
breaking of K and K* mesons. The electromagnetic effect, difference between up
and down current-quark masses and difference between up and down quark
condensates are important. We perform sum rule analyses of their masses and
decay constant differences, which are consistent with experimental values. Our
results yield Delta f_K = f_{K^0} - f_{K^\pm} = 1.5 MeV.
