For the purpose of comparative analysis, the structure of a garnet crystal activated by 1 at % of chromium, Lu3Al5O12:Cr3+, (sp. gr. Ia d, a = 11.910 Å, R w = 5.5%) and a pure garnet crystal (R w = 4.0%) is studied by neutron diffraction. It is found according to an increase in the volume of the octahedron that the Cr3+ cation is located at an octahedral position upon isomorphous substitution of the smaller Al3+ cation. Structural imperfection increases in the chromium-activated garnet, which is manifested in numerous reflections, forbidden in the space group of garnet, and the increasing divergence of refinement R w of the structure.