We have investigated site-selective doping of Ni in order to experimentally explore the roles of the Na and Co sites of NaCO2O4. The selective doping is carried out by changing preparation procedures of the oxide. Cation "substitution" is intended by incorporating Ni(NO3)2 into a Co-deficient starting mixture of Na2CO3 and Co3O4 prior to the first heat treatment. Cation "exchange" is, on the other hand, carried out by heating a mixture of Ni(NO3), and a single phase sample ofNaCo2O4, which has been prepared beforehand from a stoichiometric starting mixture. Comparison with control samples without addition of Ni(NO3)2 reveals that the "substituted" sample shows a significant decrease in the thermo power with keeping the electrical conductivity almost intact, while the "exchanged" sample shows the lower electrical conductivity with leaving the thermo power unchanged. These results strongly suggest that Ni cations occupy different sites in the Ni-doped samples; disturbing Co-Co spin exchange at the Co site, and compensating holes at the Na site.