Li-doped ZnO polycrystals with Li2CO3 concentration between 0 and 15 at.% have been prepared using a solid state reaction method. After annealing of the mixed samples at 1200 °C, Li2CO3 completely decomposed to Li2O and the lattice parameters a/c first increase with Li2CO3 concentration up to 5 at.%, and then decrease with increasing Li2CO3 concentration. This indicates that Li atoms prefer interstitial sites at low concentration and substitute Zn sites when Li concentration is beyond a certain threshold. Positron annihilation measurements were performed for undoped ZnO and Li2CO3-doped ZnO with 15 at.% samples as a function of annealing temperature. A high concentration of vacancy defects is observed in the undoped and as-prepared Li2CO3-doped samples, and they are fully removed after annealing at 1200 °C. The average positron lifetimes of all Li2CO3-doped samples after annealing at 1200 °C are the same, suggesting that Li doping has little effect on the positron lifetime in ZnO. This might be due to the invisibility of positron to Lii or LiiLiZn complex, which is further confirmed by theoretical calculation.