Abstract: Taking common CuO and ZnO powder as raw materials, ammonium citrate as dispersant to be processed with nano grinding machine, the CuO-ZnO nanocomposite preservative was prepared by wet grinding technology. The size and distribution of nanocomposite particles under different preservative concentrations were measured by ZetaPALS. The decay resistance of CuO-ZnO nanocomposite preservative was studied by referring to ‘Method of laboratory test for toxicity of wood preservatives to decay fungi’(LY/T1283-2011), and by means of the scanning electron microscope (SEM) to observe the wood decay mycelia growth after 12 weeks of treatment. The average particle diameter of the nanocomposite preservative and its leftover ratio were analyzed, the relationship between the particle diameter and the decay resistance of the timber was studied. The results showed that the average minimum particle diameter of the nanocomposite preservative was 417nm, the CuO-ZnO nanocomposite preservative showed some antibacterial property against the white and brown rot fungi, because the treated wood blocks had pretty good decay resistance and leach resistance. The first class of wood decay resistance standard could be met when 125% of the nanocomposite preservative was applied, while the average drug loading dosage was 481kg/m3 (mass loss rate was 82%), which could fit the excellent decay resistance indicators for wood product.