The crystal structure of Pr3MgNi14D18 was determined by neutron diffraction. The determined structure of Pr3MgNi14D18 consisted of 89.0% Gd2Co7-type structure and 11.0% PuNi3-type structure. The lattice parameters of a and c of Gd2Co7-type structure were refined at 0.52903(7) nm and 3.90179(1) nm. The deuterium atoms were distributed among nine deuterium sites in both the CaCu5-type and MgZn2-type cells. The D-2 occupancy in the Pr2Ni4 octahedral sites of the CaCus-type cell was the largest (0.75) when compared with the other deuterium sites (<0.49). The deuterium content of the CaCus-type cell showed 0.75 DIM, but the D/M value of the MgZn2-type cell was 1.53. The volume expansions during deuteratiOn of the CaCus-type and MgZn2-type cells were nearly equal. The cyclic hydrogenation property of Pr3MgNi14 is comparable to that of LaNis. It is inferred that the similar expansion' ehavior of the CaCus-type and MgZn2-type cells during deuteration is the origin of this cyclic stability.