Recently, Mn-based Heulser alloys with a chemical formula of Mn2YZ have been attracted attention in the field of spintronics because many of them are reported to be half-metallic ferro- or fer-ri-magnets. Although they indicate comparatively low total magnetic moment, their Curie temperature is enough high for the application. Therefore, the Mn-based Heulser alloys are thought to be potential candidates for various spintronics devices related to the spin-dependent phenomena. Especially, Mn2CoAl has been reported to have spin gapless semiconductor-type density of states [1,2], and the physical properties associated to the unique electronic state are expected to applications. The crystal structure of Mn2CoZ series is known to be Hg2CuTi-type (space group number is 216, Fig.1-1) , on the other hand, the crystal structure of Mn2NiGa, Mn2NiSn and Mn2CoSn was confirmed to be L21b-type structure (space group number is 225, Fig. 1-2) from neutron diffractions (ND) [3,4] and Mössbauer spectroscopy, respectively . Here, the symmetry is equal to L21-type structure (Fig. 1-3) whereas partially disordering exists. The disordering of the atoms often affects the electronic structure, and the investigations of the atomic configuration are very important. In the report of the phase diagram for Mn-Co-Al ternary alloy, the equilibrium state below 1273 K of the stoichiometric Mn2CoAl is in two-phase region , thus, it seems to be somewhat difficult to control precisely the degree of order and composition. In the present study, we focus on the related material of Mn2CoGa, which has been already reported to indicate half-metallic density of states from theoretical calculations , because the single phase is easily obtained down to low temperatures . Magnetic and powder ND measurements were made in the Mn2CoGa alloy, and high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) observations were complementally carried out.