The bactericidal action of silver (0) nanoparticles and amoxicillin on Escherichia coli is studied, respectively. Increasing concentration of both amoxicillin (0–0.525 mg ml−1) and silver nanoparticles (0–40 µg ml−1) showed a higher antibacterial effect in Luria–Bertani (LB) medium. Escherichia coli cells have different bactericidal sensitivity to them. When amoxicillin and silver nanoparticles are combined, it results in greater bactericidal efficiency on Escherichia coli cells than when they were applied separately. Dynamic tests on bacterial growth indicated that exponential and stationary phases are greatly decreased and delayed in the synergistic effect of amoxicillin and silver nanoparticles. In addition, the effect induced by a preincubation with silver nanoparticles is examined. The results show that solutions with more silver nanoparticles have better antimicrobial effects. One hypothesized mechanism is proposed to explain this phenomenon.