Dental composites are commonly used restorative materials; however, secondary caries due to bio?lm acids remains a major problem. The objectives of this study were(1) to develop a composite containing quaternary ammonium dimethacrylate(QADM),nanoparticles of silver(NAg), and nanoparticles of amorphous calcium phosphate(NACP), and(2) to conduct the ?rst investigation of the mechanical properties, bio?lm response and acid production vs water-ageing time from 1 day to 12 months.A 4 × 5 design was utilized, with four composites(NACP-QADM composite, NACP-NAg composite, NACP-QADM-NAg composite,and a commercial control composite), and ?ve water-ageing time periods(1 day, and 3, 6, 9, and 12 months). After each waterageing period, the mechanical properties of the resins were measured in a three-point ?exure, and antibacterial properties were tested via a dental plaque bio?lm model using human saliva as an inoculum. After 12 months of water-ageing, NACP-QADMNAg had a ?exural strength and elastic modulus matching those of the commercial control(P40.1). Incorporation of QADM or NAg into the NACP composite greatly reduced bio?lm viability, metabolic activity and acid production. A composite containing both QADM and NAg possessed a stronger antibacterial capability than one with QADM or NAg alone(Po0.05). The anti-bio?lm activity was maintained after 12 months of water-ageing and showed no signi?cant decrease with increasing time(P40.1). In conclusion, the NACP-QADM-NAg composite decreased bio?lm viability and lactic acid production, while matching the loadbearing capability of a commercial composite. There was no decrease in its antibacterial properties after 1 year of water-ageing.The durable antibacterial and mechanical properties indicate that NACP-QADM-NAg composites may be useful in dental restorations to combat caries. The objectives of this study were (1) to develop a composite containing quaternary ammonium dimethacrylate (QADM), nanoparticles of silver (NAg) rst investigation of the mechanical properties, bio? lm response and acid production vs water-aging time from 1 day to 12 months. A 4 × 5 design was utilized with four composites (NACP-QADM composite, NACP-NAg composite, NACP-QADM-NAg composite, and a commercial control composite), and water veging time periods (1 day, and 3, 6, 9, and 12 months). After each waterageing period, the mechanical properties of the resins were measured in a three-point? exure, and antibacterial properties were tested via a dental plaque bio? lm model using human saliva as an inoculum. After 12 months of water-ageing, NACP-QADMNAg had a? exural strength and elast ic modulus matching those of the commercial control (p40.1). Incorporation of QADM or NAg into the NACP composite greatly reduced bio? lm viability, metabolic activity and acid production. A composite containing both QADM and NAg possessed a stronger antibacterial capability than one with QADM or NAg alone (Po.05). The anti-bio? lm activity was maintained after 12 months of water-aging and showed no signi? cant decrease with increasing time (P40.1). In conclusion, the NACP-QADM There was no decrease in its antibacterial properties after 1 year of water-aging. The durable antibacterial and mechanical properties indicate that NACP-QADM -NAg composites may be useful in dental restorations to combat caries.