The perovskite-type LnNiO3 (Ln=Pr, Nd) were synthesized by use of an O2-HIP device. The compounds, PrNiO2.34 and NdNiO2.30 with monovalent nickel ions, could be prepared by reducing the parent perovskites in a 10%H2-90%N2 mixed gas flow at 350-degrees-C. It seemed that the composition of the reductants can be expressed as Ln3Ni3O7. The X-ray diffraction pattern for Pr3Ni3O7 was fully indexed with a tetragonal crystal lattice with a = 3.870 angstrom and c = 3.655 angstrom. That for Nd3Ni3O7 was indexed with a monoclinic cell with a = 11.041 angstrom, b = 3.765 angstrom, c = 11.770 angstrom and beta = 92.13-degrees, i.e., 3a(p)xa(p)x3a(p) supercell (a(p) : perovskite subcell). The unit cell of Nd3Ni3O7 is considered to have three Ni3+O6 octahedra and six Ni+O4 square-planes, where an octahedra layer alternates with two square-planar layers along the [101] direction.