Using the pulsed-neutron diffraction technique, we performed in situ measurements of structural ordering in the ternary alloy CuMnPt6. The diffraction patterns at various temperatures give a direct observation of a double-step ordering: disorder to Cu3Au type order as an ordering within the fundamental face-centered cubic lattice to subdivide the lattice into two sublattices formed by face-centered sites (first sublattice) and corner sites (second sublattice) at 968 degrees C; and Cu3Au type order to ABC(6) type order as an ordering within the second sublattice to subdivide the lattice further into two sublattices formed by alternating (111) planes at 746 degrees C. The order parameters for the ABC(6) type structure experimentally estimated by the method of static concentration waves indicate that the primary ordering developed almost completely, but the secondary ordering remained incomplete.