This study presents a promising ultraviolet (UV)-curable epoxy resin formulation with improved flame-retardant properties. The formulation is based on the cycloaliphatic epoxide 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (ERL4221) and a novel silicon, phosphorous containing flame-retardant additive. The additive, 1,3,5,7-tetramethyl-1,3,5,7-tetra 2-(6-oxido-6-H-dibenzo(c,e) (1,2)oxaphosphorin-6-yl) ethylcyclotetrasiloxane (DOPO-SiD), was synthesized by the addition reaction of 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane (D4Vi) with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). Formulations containing the cycloaliphatic epoxy resin ERL4221 and the flame-retardant DOPO-SiD additive were prepared in various concentrations and crosslinked by UV irradiation. The effects of DOPO-SiD and photoinitiators, such as the cyclopentadienyl iron complex of carbazole (In-Fe) and diphenyl-(4-(phenylthiol) phenyl) sulfonium hexafluorophosphate (In-S), on the flame-retardant properties and thermal stabilities of UV-cured ERL4221/DOPO-SiD composites were investigated with limiting oxygen index, UL-94 vertical test, and thermogravimetric analysis, respectively. The results showed that DOPO-SiD can increase the thermal stabilities of the ERL4221/DOPO-SiD. The char yield was improved when DOPO-SiD and In-Fe were simultaneously used. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40011.