Transmission electron microscopy(TEM) investigation on the phase decomposition of B2-ordered (Ni, Co)Al supersaturated with Ni and Co has revealed the precipitation of (Ni, Co)₂Al which has not been expected from the reported equilibrium phase diagram. The (Ni, Co)₂Al phase has a hexagonal struture and takes a rod-like shape with the long axis of the rod parallel to the <111> directlions of the B2 matrix. By aging at temperatures below 873K, a long period Superlattice Structure appears in the hexagonal (Ni, Co)₂Al Phase. The orientation relationship between the (Ni, Co)₂Al Precipitates and the B2-(Ni, Co)Al matrix is found to be(0001)_P // (111)_(B2) and [1￣21￣0]_P // [1￣10]_(B2), Where the suffix p and B2 denote the (Ni, Co)₂Al precipitate and the B2-(Ni, Co)Al matrix. respectively. (Ni, Co)Al hardens appreciably by the fine precipitation of the (Ni, Co)₂Al phase. Energy dispersive spectroscopy was used to analyze the compositions of each phase formed in B2-(Ni, Co)Al.