The aim of this ex vivo study was to evaluate the adherence capacity of Streptococcus mutans after being exposed to three different sizes of silver nanoparticles on healthy human dental enamel. Three different sizes of silver nanoparticles (9.3, 21.3 and 98nm) were prepared, characterized and an adherence testing was performed to evaluate their anti-adherence activity on a reference strain of S. mutans on healthy dental enamel surfaces. Colony-Forming Unit count was made for adherence test and light microscopy, atomic force microscopy and scanning electron microscopy were used to compare qualitative characteristics of S. mutans. 9.3nm and 21.3nm groups did not show differences between them but statistical differences were found when 9.3nm and 21.3nm groups were compared with 98nm and negative control groups (p<0.05). Microscopy analysis shows a better inhibition of S. mutans adherence in 9.3nm and 21.3nm groups than the 98nm group when compared with control group. Silver nanoparticles showed an adherence inhibition on S. mutans and the anti-adherence capacity was better when silver nanoparticles were smaller.