Objective: The development of new longer-lasting composite resins is an urgent public health need. It has been shown that surface roughness of composite resins is increased by Streptococcus mutans biofilm in vitro and further that incorporation of small amounts of antibacterial nanoparticles (polyethyleneimine [PEI]) into composite resins renders a strong antibacterial effect against S mutans biofilm. The present study tested the hypotheses that incorporation of PEI nanoparticles into composite resins prevents the increase of surface roughness caused by S mutans biofilm and that PEI incorporation into composite resin has a long-lasting antibacterial effect. Method and Materials: Composite resin incorporating PEI nanoparticles was characterized using contact angle goniometry, X-ray photoelectron spectroscopy (XPS), and SEM. Six-month-aged samples were tested for antibacterial effect against S mutans using the direct contact test. Surface roughness following 1 month of bacterial challenge was depicted using atomic force microscopy (AFM). Results: Contact angle increased following PEI incorporation, and XPS revealed surface iodide and nitrogen elements. Direct contact test results showed that 6-month-aged composite resins incorporating PEI nanoparticles completely inhibited S mutans growth (P .05). AFM analysis showed an increase in root mean square roughness following bacterial challenge in composite resin samples (P .05); no effect was depicted in samples incorporating PEI. Conclusion: Changing the surface properties of composite resins by incorporating PEI antibacterial nanoparticles may improve their clinical performance both by inhibiting bacterial growth and by preventing changes in the surface roughness. Keywords: biofilm, composite resins, nanoparticles, polyethyleneimine