An Evaluation of fπ from αs (Mz) in the Schwinger-Dyson Approach

We examine the scale ambiguity of the improved ladder Schwinger-Dyson equation for the dynamical chiral symmetry breaking in QCD. The use of the effective coupling is proposed to reduce size of the ambiguity. The pion decay constant f_π is then calculated from the high energy QCD gauge coupling α ^{/line {MS}}_s(M_Z). The result agrees with its experimentally observed value and ascertains the validity of the use of the improved ladder approximation in QCD more firmly than considered before.