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Novel c-6 position oxidized corn starch (OST) with high carboxyl content (26.3-54.5%) was prepared by a green method, using hydrogen peroxide as the oxidant. The as-obtained OSTs were then used as flame-retardant carbon sources with microencapsulated ammonium polyphosphate (MFAPP) in epoxy resin (EP). Compared to EP, the obtained EP/MFAPP/OST composites exhibit significantly enhanced flame retardancy. The introduction of only 6.25 wt % OSTs and 6.25 wt % MFAPP results in remarkably increased limiting oxygen index and decreased heat release rate, and all composites can reach UL94 V-0 rating. Thermogravimetric analyses and cone calorimetry results suggest both OSTs and MFAPP have good catalytic charring effects, and the increased carboxyl content benefits the char formation of the composites. Because of the formation of compact char on the sample surface during combustion, the transfer of oxygen, heat, and flammable gas products is inhibited; the flame retardancy of EP/MFAPP/OST composites is thus remarkably enhanced. [ABSTRACT FROM AUTHOR]