The ferromagnetic Al2O3-based cermets with different ratios of Co and Co-50Ni alloys were successfully prepared by combustion synthesis in thermal explosion (TE) mode. The reaction process, microstructure, and magnetic property of cermets were investigated. The relative density of cermets can be over 95% via uniaxial loading at the time of ignition when the cermets are hot and ductile. In Al2O3-Co cermets, beta-Co and alpha-Co co-exist at room temperature with average size of less than 10 mu m and disperse homogeneously in the matrix, while in Al2O3-(Co-50Ni) cermets, the network-like Co-50Ni alloy can infiltrate into the boundary gaps of Al2O3 particles. The ferromagnetic Co and Co-50Ni alloys are responsible of the magnetic properties of Al2O3-based cermets. The saturation magnetization strongly depends on the magnetic characteristics and ratios of ferromagnetic phases. Al2O3-(Co-50Ni) cermets have soft magnetic properties with high magnetic susceptibility and low coercive force.