Does a private company that is 100% financed by a bank loan have a WACC equal to the interest rate? When estimating a company's Cost of Debt for a Weighted Average Cost of Capital (WACC) calculation we normally look into its bond yield. But for a private company with 100% debt capital structure, will the cost of debt be simply equal to the interest rate? If not why?

Thanks.
Let's address your question in a canonical manner. Say that:


the required rate of return of (private) investors is denoted by $r$.
the interest rate or the cost of debt is denoted by $r_D$ (naturally $r_D &lt; r$)
the share of debt in the capital structure of the company is denoted by $X\%$
the corporate tax rate is denoted by $T_c$


It follows that the average cost of capital (average over equity and debt), $r'$, is such that

$r' = X\% r_D (1-T_c) + (1-X\%) r$



In your case, $X\% = 100\% = 1$, then $r' = r_D(1-T_c)$.


  [...] will the cost of debt be simply equal to the interest rate? If not why?


No. Because of $T_c$, the corporate tax rate being potentially non-zero.