Abstract: The study offers a theorem on the properties of the bond price; it is formulated and proved mathematically-under the same conditions as established theorems on the bond valuation. The theorem concerns the previously unstudied dependency of bond price variation at changing terms to maturity on the level of required yield. For bonds that are sold at a discount the maximums of the absolute and relative bond price changes are found. The assertions proved in the theorem are confirmed by the proper calculations.