In this study, size-controlled single-crystal copper oxide nanosheets (CuO NSs) were fabricated by a microwave-assisted chemical method. Next, their nanocomposites with high-density polyethylene (HDPE) were fabricated. These nanocomposites were further reinforced with carbon nanotubes (CNTs) of oil fly ash. The produced CuO NSs along with their polymer nanocomposites were evaluated as antimicrobial agents against Escherichia coli, Bacillus subtilis, and yeast Saccharomyces cerevisiae. Significant microbial-growth inhibition was observed. This inhibition correlated with oxygen defects, free Cu2+ ion release, and a change in the lattice constant c. These factors were found to increase with the increased molar ratio of the precursors and might be responsible for formation of additional reactive oxygen species.Moreover, adding 1 wt% of CuO NSs and 0.5 wt% of CNTs enhanced the mechanical properties of HDPE by 20%. These results indicate that it is possible to design polymer nanocomposites that have both antimicrobial and enhanced mechanical properties for various applications, mainly in the healthcare sector. [GRAPHICS] .