Disclosed herein is a symmetric cyclic phosphonate compound represented by the following Formula 1, a method of preparing the same and a flame retardant styrenic resin composition including the same: wherein R1 and R2 are each independently hydrogen, C1-C6 alkyl or C6-C20 aryl. The styrene resin composition employing the symmetric cyclic phosphonate compound exhibits good flame retardancy and impact strength, does not release halide gas during preparation or combustion of the resin composition, and thus is environmentally friendly.