Recently we have modifided the theory of Nobel Prize winners Modigliani and Miller (MM theory), which is a perpetuity limit case of the general theory of capital cost and capital structure—Brusov–Filatova–Orekhova theory (BFO theory), into two ways: we apply it for rating methodologies needs and later we generalized it for the case of advance payments of tax on profit, which is widely used in practice (MMM theory) (Brusov et al., Mathematics 9(11):1286, 2021). In current Chapter we use the Modified Modigliani–Miller theory (MMM theory) and apply it for rating methodologies needs. The financial “ratios” (main rating parameters) were introduced into MMM theory. The dependence of the weighted average cost of capital (WACC), which plays the role of discount rate in financial flows discounting in rating methodologies, on coverage and leverage ratios is analyzed. Obtaind results will help improve the existing rating methodologies. During a couple years we have suggested a new approach to rating methodology of non–financial issuers, as well for project rating (for long–term projects as well as for projects of arbitrary duration) (Brusov, J Rev Glob Econ 7:i–vi, 2018; Brusov, J Rev Glob Econ 7:i–v, 2018; Brusov et al., Modern corporate finance, investments, taxation and ratings, Springer, 2018; Brusov et al., J Rev Glob Econ 7:37–62, 2018; Brusov et al., J Rev Glob Econ 7:88–103, 2018; Brusov et al., J Rev Glob Econ 7:63–87, 2018; Brusov et al., J Rev Glob Econ 7:104–122, 2018; Brusov et al., J Rev Glob Econ 7:360–376, 2018; Brusov et al., Modern corporate finance and investments, Knorus Publishing House, 2018; Brusov et al., J Rev Glob Econ 8:437–448, 2019; Brusov et al., J Rev Glob Econ 9:257–267, 2020; Filatova et al., J Rev Glob Econ 7:645–661, 2018). The key factors of a new approach are: (1) The adequate use of discounting of financial flows virtually not used in existing rating methodologies, (2) The incorporation of rating parameters (financial “ratios”) into the modern theory of capital structure (Brusov–Filatova–Orekhova (BFO) theory) (Brusov et al., Modern corporate finance, investments and taxation, Springer, 2015; Brusov, J Rev Glob Econ 7:i–vi, 2018; Brusov, J Rev Glob Econ 7:i–v, 2018; Brusov et al., Modern corporate finance, investments, taxation and ratings, Springer, 2018; Brusov et al., J Rev Glob Econ 7:37–62, 2018; Brusov et al., J Rev Glob Econ 7:88–103, 2018; Brusov et al., J Rev Glob Econ 7:63–87, 2018; Brusov et al., J Rev Glob Econ 7:104–122, 2018; Brusov et al., J Rev Glob Econ 7:360–376, 2018; Brusov et al., Modern corporate finance and investments, Knorus Publishing House, 2018; Brusov et al., J Rev Glob Econ 8:437–448, 2019; Filatova et al., J Rev Glob Econ 7:645–661, 2018) and into its perpetuity limit. Recently we have generalized the Modigliani and Miller theory for a more realistic method of payments of tax on profit payments: for the case of advance payments of tax on profit, which is widely used in practice (Brusov et al., J Rev Glob Econ 9:282–292, 2020). Modigliani–Miller theory accounts these tax payments as annuity–immediate, while in practice these payments are making in advance and thus should be accounting as annuity–due. We have shown that this generalization leads to some important consequencies, which change seriously all the main statements by Modigliani and Miller. In current Chapter we use the modified Modigliani–Miller theory (MMM theory) and apply it for rating methodologies needs. A serious modification of MMM theory in order to use it in rating procedure has been required. The financial “ratios” (main rating parameters) were introduced into MMM theory. The necessity of an appropriate use of financial flows discounting in rating methodologies is discussed. The dependence of the weighted average cost of capital (WACC), which plays the role of discount rate, on coverage and leverage ratios is analyzed. Obtained results make possible to use the power of this theory in the rating and create a new base for rating methodologies, by other words this allows develop a new approach to methodology of rating, requiring a serious modification of existing rating methodologies.