A dental composite composition is disclosed that includes a polymerizable resin, filler particles, and at least one polymerizable stable radical. A variety of polymerizable stable radicals may be employed, including those that have a 2,2,6,6-tetramethylpiperidinyl-1-oxyl moiety. Compositions as described herein exhibit excellent mechanical strength, hardness, and flexural modulus, while also significantly decreasing shrinkage stress caused by polymerization.