The microstructural changes of both solution heat treated (ST) and cold worked (CW) MP159 alloy during aging were investigated by optical microscopy and transmission electron microscopy (TEM). The results indicate that both solution heat treated (ST) and cold worked (CW) MP159 alloy could be hardened by aging. This aging induced hardening is attributable to the precipitation of a very finely dispersed ordered cubic γ′ phase particles with only several nanometres in diameter. The hexagonal close packed (HCP) ε -martensite precipitated during aging of both CW MP159 and CW MP35N, and the hexagonal ordered Co3Mo phase of the DO19 structure type precipitated during aging of CW MP35N, have not been found in the present investigation.