Copper (Cu) nanoparticles with mean diameters from 44 to 97 nm were synthesized by electroless deposition at 353 K in water. The diameter of the Cu nanoparticles was tailored by changing the amount of surfactant (gelatin) and reducing agent (hydrazine). Smaller Cu nanoparticles were generally prepared at low concentrations of gelatin and high amounts of hydrazine (N2H4). In fact, cuprous oxide (Cu2O) was also precipitated in the solution when the concentration of N2H4 was not sufficient. On the other hand, in situ mixed potential measurement showed that gelatin also influenced the rate of Cu deposition. The Cu nanoparticles displayed high antimicrobial activity against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus). Then again, S. aureus showed higher susceptibility for the Cu nanoparticles.