Abstract: CoCr_0.4NiSi_0.3B_y mesentropic alloy with different B content (y=0.001, 0.004, 0.007, molar fraction) were prepared by melting, and hot compression tests at different temperatures (750, 850, 950, 1 050 ℃) and 0.01 s⁻¹ strain rate were carried out. The effects of B content and hot compression temperature on microstructure, phase composition and hot deformation behavior of the test alloy were studied. The results show that dynamic recrystallization occurred in the alloy with different B content after hot compression at different temperatures, and the phase consisted of face-centered cubic matrix phase, a small number of hexagonal closely packed and tetragonal phases. With the increase of B content, the intensity of the hexagonal closely packed and tetragonal phase diffraction peak increased, the diffraction peak of (111) crystal plane of face-centered cubic phase became stronger than that of (200) crystal plane. The hardening of the second phase containing B existed during the hot compression at medium temperature of 750 ℃, and the phase transformation after hot compression at high temperature of 1 050 ℃, which was the high temperature softening mechanism of the alloy. With the increase of B content or the decrease of hot compression temperature, the peak flow stress increased. With the increase of B content or the increase of hot compression temperature, dynamic recrystallization grain growed, the average grain size of the alloy increased and the volume fraction of twin crystals decreased.