Abstract: The variation in hardness of 80Co-20Ti intermetallic compounds with aging time was investigated, and the change in yield strength of aged 80Co-20Ti intermetallic compounds undergoing aging and containing precipitates with temperature was tested. Transmission electron microscope (TEM) observations reveal that the precipitates are L1₂Co precipitates with simple fee crystal structure. Although the hardness of the precipitates is less than that of Co₃Ti matrix phase, the Co₃Ti hardens appreciably by fine precipitation of disordered fee Co-rich phase. The precipitates are perfectly coherent with the matrix lattice at the beginning of aging. The shape of the precipitates is platelet and is nearly parallel to the[100]planes of the L1₂-ordered matrix. It is indicated that the interaction between L1₂Co precipitates and deformation dislocations is attractive interaction and this type of interaction plays an important contribution to the hardening. The strengthening mechanism deduced from the attractive interaction between the soft precipitation and hard matrix was also discussed.