Neutron diffraction measurements were carried out on H/D isotopically substituted water in the low-density supercritical condition (T = 673 K, P = 26.3 MPa, and rho=0.0068 molecules.angstrom(-3)) in order to obtain direct information on the intermolecular partial structure functions, g(HH)(inter)(r), g(OH) (inter)(r), and g(OO)(inter)(r). In correspondence to the high-density supercritical water previously reported, the intermolecular nearest neighbor peaks in g(HH)(inter)(r), g(OH)(inter)(r), and g(OH)(inter)(r) are diffuse compared with the ambient ones. The nearest neighbor O center dot center dot center dot O distance (3.3 angstrom) and its coordination number (2.6) were determined from the observed g(OO)(inter)(r). These results indicate that the orientational correlation between neighboring water molecules is considerably lost in low-density supercritical water. Small clusters involving a few water molecules are preferentially formed in low-density supercritical water, which is consistent with results obtained by previous IR and NMR studies.