Objectives: Temporary restorative materials (TRMs) often rapidly lose their dimensional stability and antibacterial properties after exposure to humidity and bacterial infection. Quaternary ammonium polyethyleneimine (QPEI) nanoparticles (NP) are long-lasting, stable, biocompatible, and nonvolatile antibacterial polymers. In the present study, we incorporated QPEI NP into standard TRMs and examined their influence on dimensional stability and their ability to reduce bacterial leakage. Method and Materials: A modified split-chamber model was used in vitro to test calcium sulfate-based and zinc oxide-eugenol- based TRMs (Coltosol and IRM, respectively). Both materials were tested with and without 2% wt/wt incorporated QPEI NP for fluid and bacterial leakage. Results: The calcium sulfate-based TRM displayed the lowest microleakage and highest antibacterial resistance. Two-way A NOVA analysis of the fluid transport test results showed that incorporation of 2% wt/wt QPEI NP significantly increased the sealing ability of both TRMs (P .01). A nalysis of survival curves by the Kaplan-Meier method showed that the calcium sulfate-based TRM with 2% wt/wt QPEI NP survived the bacterial load significantly more effectively than did the zinc oxide-eugenol-based TRM (P .0001). Conclusion: Incorporation of 2% w/w QPEI NP may prominently improve the sealability and the antibacterial properties of TRMs. TRMs incorporating antibacterial nanoparticles may be clinically advantageous for sealing the endodontic access cavity to avoid reinfection of the root canal system during endodontic treatment. Keywords: nanoparticles, polyethyleneimine, temporary restorative materials