We calculate the low-energy parameter S in QCD, which is also known as L_10, and the pion decay constant f_π using inhomogeneous Bethe-Salpeter equation in improved ladder approximation. To extract these quantities we calculate the "V-A" two-point function, Π_VV(q^2) - Π_AA(q^2), in space-like region. We obtain S = 0.43 ∼ 0.48, which is about 30 the experimental value. The calculated f_π is well consistent with the result by solving the homogeneous Bethe-Salpeter equation for pion. We also

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... ate S parameter in SU(3) gauge theory with N_D doublets of fermions in connection with walking technicolor model, and find that the value of S/N_D hardly depends on N_D.