Objectives: To investigate the effects of N-vinylcaprolactam (NVC)-containing terpolymers on the fracture toughness, microhardness, and flexural strength of conventional glass-ionomer cements (GIC). Methods: The terpolymer of acrylic acid (AA)-itaconic acid (IA)-N-vinylcaprolactam (NVC) with 8:1:1 (AA:IA:NVC) molar ratio was synthesized by free radical polymerization and characterized using 1H NMR and FTIR. Experimental GIC samples were made from a 50% solution of the synthesized terpolymer with Fuji IX powder in a 3.6:1 P/L ratio. Specimens were mixed and fabricated at room temperature. Plane strain fracture toughness (KIc) was measured in accordance with ASTM Standard 399-05. Vickers hardness was determined using a microhardness tester. Flexural strength was measured using samples with dimensions of 2 mm × 2 mm × 20 mm. For all mechanical property tests, specimens were first conditioned in distilled water at 37 °C for 1 day or 1 week. Fracture toughness and flexural strength tests were conducted on a screw-driven universal testing machine using a crosshead speed of 0.5 mm/min. Values of mechanical properties for the experimental GIC were compared with the control group (Fuji IX GIC), using one-way ANOVA and the Tukey multiple range test at α = 0.05. Results: The NVC-modified GIC exhibited significantly higher fracture toughness compared to the commercially available Fuji IX GIC, along with higher mean values of flexural strength and Vickers hardness, which were not significantly different. Significance: It was concluded that NVC-containing polymers are capable of enhancing clinically relevant properties for GICs. This new modified glass-ionomer is a promising restorative dental material. © 2010 Academy of Dental Materials.