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Neutron powder diffraction patterns of deuterated sodium cobaltate, which shows superconductivity with Tc of about 4.5 K, show conspicuous diffuse maxima presumed to originate from the deuterated sodium part in addition to sharp reflections due to the CoO2 part. The structure is treated as an interpenetration of two layered subsystem structures, the CoO2 part (a1 = 2.83 Å) and the deuterated sodium part (a2 = 3.26 Å). A model of the short-range-order stacking of the deuterated sodium part is expressed using four possible configurations. The sequences in which neighbouring sheets have the same configuration do not occur, and the other sequences occur with the same probability. Diffuse scattering intensities simulated using the matrix method satisfactorily explain the neutron diffraction patterns. [ABSTRACT FROM AUTHOR]