The structure, structural relaxation and crystallization of metallic glass Zr78Co22 are investigated by the small angle X-ray diffraction, the wide angle X-ray diffraction and the field ion microscopy, together with the measurements of the superconducting critical temperature and the microhardness. The results indicate that there exists obviously the amorphous phase separation in the as-quenched state of the metallic glass Zr78Co22. The compositions of two segregated amorphous phases are similar to pure Zr and Zr4Co respectively. In the process of structural relaxation, the changes of the toplogical short range order are essential and controlled by the nucleation-growth mechanism. When the number of nucleation is sufficient, the segregation grow homogenously and identically.