Since the co-doping method is a key technique for the realization of p-type ZnO films, we grew samples by the co-doping of Li and Ni impurities using a pulsed laser deposition technique to investigate the formation of a possible acceptor level. The effect of the oxygen gas pressure on the physical properties is discussed. The electrical and optical properties of the films were investigated through Hall and photoluminescence (PL) measurements. Although p-type conduction was not observed, we were able to observe the change in conduction type from n-type to insulating upon increasing the oxygen pressure. PL spectra of co-doped samples were investigated at 10 K by comparison with those of mono-doped samples. Co-doping drastically changes the PL spectral shape and reveals additional peaks at 2.4 and 3.0 eV, which could not be observed in the spectra of mono-doped samples. The experimental results showed that the acceptor level was indeed formed by the co-doping of Li and Ni.