In this paper, the (Fe_0.25Co_0.25Ni_0.25Cr_0.125Mn_0.125)_100-xB_x (x=8-16, at.%) alloys were prepared by vacuum arc melting, ejection casting into a copper mold and single-roller melt-spinning. The phase composition, microstructure evolution and mechanical properties of the high entropy alloys with different B content were studied under different casting conditions. The results show that the high entropy alloys can be transformed from fcc phase to amorphous phase by controlling B content and cooling rate. In the same casting process, the grains can be refined by adding B content, so that the strength and hardness of the alloys increase with the increase of B content.