Polycrystalline MnNiCuFeO was implanted by B+, P+ and Si+ ion beams and thermally annealed. The structure and electrical properties of the sample were measured using SEM, Microprobe (MP), Low Frequency Impedance Analyzer (LFIA) and Spreading Resistance Probe (SRP). The results show that the resistance of grain boundaries is much higher than that of grains. The spreading resistance of the implanted samples is lower by factor of 2 than that of the unimplanted ones. The ratio of the real part Rs (grain effect) to imaginary part Xs (grain boundary effect) decreases with ion beam implantation. From these results, we came to the conclusion that the behavior of the grain boundaries is important to the bulk properties of polycrystalline MnNiCuFeO.