Investigation is performed for alloys with boron content of 9.0–15.0% wt., the rest is iron. To determine the structural state of alloys, we use the microstructure analysis, as well as the differential thermal and the X-ray diffraction analyses. In the paper we determine the mechanism of Fe$_5$B$_3$ boride formation and show that formation of this phase occurs in Fe–B alloys with boron content of 9.0–15.0% wt. as a result of peritectic transformation L + FeB $\rightarrow$ Fe$_5$B$_3$. As found, an annealing of Fe–B system alloys with boron content of 10.5% wt. at a temperature of 1473 K for four hours and a cooling with a rate of 10$^2$ K/s lead to decrease in the containing of FeВ monoboride and Fe$_2$В boride. There is an increase in the volume fraction of Fe$_5$B$_3$ boride by the transformation in a solid state FeB + Fe$_2$В $\rightarrow$ Fe$_5$B$_3$. As shown, the Fe$_5$B$_3$ phase forms at the temperature of 1650 K, and at 1410 K it loses the thermodynamic stability with formation of FeВ monoboride and Fe$_2$В boride. The accounting for two first order of smallness of the high-temperature expansion for thermodynamic potential of Fe$_5$B$_3$ boride in a binary alloy of the Fe–B system enables to derive temperature dependence of thermodynamic stability of this phase. For the first time it is demonstrated that decrease of Fe$_5$B$_3$ phase stability at 1423 K shows that there its decomposition is started with formation of other phases.