Abstract: To investigate the relationships between quenching cooling ways and microstructure transformation of the high boron medium carbon alloy (HBMCA), the samples were prepared by quenching HBMCA at 1 000 ℃ in three media, i.e., air, oil and water, followed by tempering at 500 ℃. The microstructure transformation of the samples was analyzed by using X-ray diffraction. The hardness and high temperature tribological performance were investigated. The results show that the matrix of air cooled sample consisted of tempered sorbite, small amount of pearlite and M₃(B,C) precipitates. The oil cooled and water cooled samples were composed of tempered sorbite as the matrix and MB and M₂B type hard phases. As the cooling rate increased,volume fraction of the hard phase slightly decreased while the hardness and wear resistance increased.The main wear mechanism of air cooled sample was oxidation wear. Oxidation spalling was alleviated for oil cooled sample. For water cooled sample, the wear mechanism was mainly caused by the combined action of furrow deformation and oxidation-removal.