The paper is devoted to the study of crystallization processes and the effect of the phase composition of rapidly quenched high-entropy alloys (Fe0.25Co0.25Ni0.25Cr0.125V0.125)100- x B x , where x = 14, 17, 22 and 25 on microhardness. Using differential scanning calorimetry, X-ray diffraction and transmission electron microscopy, the evolution of the structure of the studied alloys during crystallization was established. It is shown that in alloys with a boron content of 14-17%, the formation of a two-phase mixture of FCC and BCC phases, without a residual amorphous matrix, is observed during crystallization. The possibility of forming a boron-supersaturated solid solution with an FCC lattice, which undergoes decomposition at high annealing temperatures with the formation of dispersed particles of Me3B2 borides, is demonstrated. Increasing the boron content in the composition leads to the formation of a dispersed eutectic mixture of a boron-supersaturated FCC phase and Me23B6 boride in the structure during annealing, followed by the decomposition of the supersaturated solid solution to form Me3B2 borides. The studied alloys exhibit microhardness of 700-2000 HV, depending on the structural state. It is shown that the formation of Me3B2 borides and simultaneous grain growth leads to a decrease in the microhardness of the alloys. The formation of a dispersed mixture of a boron-supersaturated FCC phase and Me23B6 boride ensures maximum microhardness values.