Abstract: Ageing process and its hardness change from β to α+Ti₂Co of alloy Ti-12Co-5Al were investigated. At 450℃,the amount of transformation increases with ageing time and the boundary between the transformation product and the matrix changes from coherent to incoherent. The maximum hardness is achieved at around 320 minutes. With the same ageing time of 20 minutes, the same thing happens as above with increasing temperatures and the hardness peaks at around 600℃.The addition of element aluminum slows down the kinetics of ageing transformation in addition to its strengthening effect.