We propose a non-perturbative calculation scheme which is based on the semi-classical approximation of QCD and can be used to evaluate quantities of interest in hadronic physics. As a first application, we evaluate the mass of the pion and of the nucleon. Such masses are related to a particular combination of Green functions which, in some limit, is dominated by the contribution of emph{very small-sized} instantons. The size distribution of these pseudo-particles is determined by the t Hooft tunneling amplitude formula and therefore our calculation is free from any model parameters. We prove that instanton forces generate a light pion and a nucleon with realistic mass ($M_n sim 970 MeV$). In connection with sum-rules approaches, we discuss the overlap of instantons with pion and nucleon resonances.
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