Background: Although, mineral trioxide aggregate (MTA) and new experimental cement (CEM) are good root filling cements, but had no or low antimicrobial activities. The aim of this study was to evaluate the effects of addition of silver nanoparticles (SNP) to these two cements on antimicrobial effects against five most dental infection related microorganisms. Material and Methods: Two suspensions of 100 and 200 ppm of SNP were prepared and 180 μl of microbial suspension with 1.5 × 10 8 CFU/ml of each respected microorganisms were re-suspended in deionized water or each of SNP suspensions. After that, 60 μg of MTA and CEM were added to each tube. In one tube, the mixture of all above mentioned microorganisms were added as a source of microorganism. Colonies were counted after 0, 24, 48, 72 and 96 hours intervals of incubation at 35°C on blood agar for evaluation of antimicrobial efficacy. Results: MTA and CEM had antibacterial activities on all microorganisms’ strains except for Enterococcus faecalis and mixture group. MTA had better antibacterial activity than CEM but the difference was not significant ( p >0.05). The combination of SNP with two cements resulted in significantly higher antimicrobial activities ( p <0.05). Also, there was no statistically significant difference between two SNP concentrations ( p >0.05). Conclusions: Mixture of MTA and CEM with different concentrations of SNP significantly increased the antibacterial activity