Risky debt valuation differs greatly from that of no risk so that under uncertainty of income flower and discount rate, usually we can not get a kind of resonlution to express the type. With no account of the discount rate, we are absorbed in property value influence under uncertainty. As generalizing the theory of option pricing, contingent claims analysis can handel debt valuation, and sometime give closed form expressions.Two formula obtained by Merton and Black&Cox, are Compared and different conditions are discussed. At last, some comment on formula of debt valuation is given.