A novel phosphorous-silicone-nitrogen ternary flame retardant (FR), [(1,1,3,3-tetramethyl-1,3-disilazanediyl)di-2,1-ethanediyl]bis(diphenylphosphine oxide) (PSiN) was synthesized with high yield via a one-step procedure by the reaction of diphenylphosphine oxide and vinyl-terminated silazane with triethylborane as the catalyst. The chemical structure of the target compound was confirmed by nuclear magnetic resonances, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and Flourier transform infrared measurements. The developed PSiN FR was applied in the flame retardancy of o-cresol novolac epoxy (CNE)/phenolic novolac (PN) hardener system. Effects of PSiN on the processability, thermal properties, especially the flame retardancy properties of the composites were investigated. Experimental results revealed that addition of PSiN improved the flowability of the CNE/PN systems, while the thermal stability of the epoxy thermosets was maintained. Meanwhile, the incorporation of PSiN was in favor of the formation of char during the thermal degradation process of the epoxy thermosets. The limited oxygen index of the epoxy system increased along with the PSiN content. An UL 94 V-0 FR rating was achieved when the weight content of PSiN in the epoxy composites reached 20 wt %. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40412.