A novel flame retardant (DPA-SiN) containing phosphorus, nitrogen and silicon elements at the same time was synthesized. 9-(9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide)-propanoic acid (DPA) synthesized through simple addition reaction of 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and acrylic acid (AA) was introduced into N-beta-(aminoethyl)-gamma-aminopropyl methyl dimethoxysilane/dimethylsiloxane copolymer (SiN) to form a novel flame retardant (FR). The structure of DPA and DPA-SiN were characterized by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. After blend with PC/ABS, the flame-retardant properties of the PC/ABS/DPA-SiN blends are estimated by Limiting Oxygen Index (LOI) values and CONE Calorimeter, while thermal stabilities are investigated through Thermo Gravimetric Analysis (TGA). The morphological structures of the char formed after PC/ABS/DPA-SiN burnt in LOI test were investigated by Scanning Electron Microscopy (SEM).