The metastable crystalline (MX) phases were achieved by in situ annealing in the sputtering system of CoxCu1-x (x = 0.25, 0.5) nano-multilayers. Then the stability of the MX phase was studied by in situ annealing in transmission electron microscopy (TEM). Interestingly, two MX phases of fcc structure, with lattice parameters of around 4.01 and 3.31 Angstrom, respectively, were formed in the Co50Cu50 sample and were quite stable under thermal annealing up to 600 degrees C. A unique fcc phase with a = 3.41 Angstrom was obtained in the Co25Cu75 sample, grew with increasing temperature and transformed to a perfect single crystal at 600 degrees C, XRD and thermomagnetic measurements also confirmed the TEM results.