Neutron diffraction technique was applied to PrNiO2.34 and the unit cell of PNiO2.34 was reinvestigated. The diffraction peaks could be assigned by the monoclinic cell of a=8.656(6) Angstrom, b=3.655(2) Angstrom, c=5.442(4) Angstrom and beta=71.50(7)degrees. The cell can be expressed as root5 x I x root2 of the fundamental perovskite. This suggests. PrNiO2.34 should be the compound Pr3Ni3O8. The existence of Ln(m+n)[Ni3+O6/2](m)[Ni+O4/2](n) homologous series can be proposed, where m sheets of octahedral layers alternate with n sheets of square planar layers along the [110] direction of the fundamental perovskite-type lattice. All the compounds appeared in the reduction processes of the perovskite-type LnNiO(3) (Ln=La, Pr, N), La3N3O8, La2Ni2O5 and Ln(3)Ni(3)O(7) (Ln=Pr, Nd) belong to the homologous series.