Objective. In this study, three dimethacrylate quaternary ammonium compounds N,N-bis[2-(3-(methacryloyloxy)propanamido)ethyl]-N-methyldodecyl ammonium iodide (QADMAI-12), N,N-bis[2-(3-(methacryloyloxy)propanamido)ethyl]-N-methylhexadecyl-ammoniumiodide (QADMAI-16), and N,N-bis[2-(3-(methacryloyloxy)propanamido)ethyl]-N-methyl-octadectyl ammonium iodide (QADMAI-18) were synthesized and proposed to be used asantibacterial and radio-opaque agents in dental resin.Methods. All QADMAIs were synthesized through a 2-steps reaction route, and theirstructures were confirmed by FT-IR and1H NMR spectra. Antibacterial activities againstStreptococcus mutans (S. mutans) of QADMAIs were measured by agar diffusion test. EachQADMAI was mixed with TEGDMA (50/50, w/w) and photoinitiation system (0.7 wt% of CQand 0.7 wt% of DMAEMA) to form resin system. Degree of monomer conversion (DC) wasdetermined by FT-IR analysis. The flexural strength (FS) and modulus (FM) of the polymerwere measured using a three-point bending set up. Radiograph was taken to determine theradio-opacity of the polymer, and aluminum step-wedge (0.5–4 mm) was used as calibrationstandard. Surface charge density was measured using fluorescein binding. A single-speciesbiofilm model with S. mutans as the tests organism was used to evaluate the antibacterialproperty of the polymer. Bis-GMA/TEGDMA resin system was used as control material in allof the tests.Results. FT-IR and1H NMR spectra showed that the structures of QADMAIs were the sameas designed. ANOVA analysis revealed that antibacterial activity of QADMAI decreased withthe increasing of alkyl chain length (p < 0.05). QADMAI containing polymers had higher DC(p < 0.05) but lower FS and FM (p < 0.05) than control polymer. Alkyl chain length had no influ-ence on DC (p > 0.05), but FS and FM of QADMAI-12 containing polymer were better than thoseof QADMAI-16 and QADMAI-18 containing polymers (p < 0.05). QADMAI containing poly-mers had much better radio-opacity than control polymer (p < 0.05), and the radio-opacity