Phosphorus flame retardants have been considered to be ineffective for polycarbonates (PCs). In light of the recognition that the high efficiency of sulfonate salts is due to the base catalyzed decomposition of polycarbonate, a tertiary amine group is incorporated into a 10-dihydro-9-oxa-10-phosphaplienanthrene-10-oxide (DOPO) derivative which was synthesized from DOPO, paraformatdehyde, and piperazine (DFPZ). DFPZ shows excellent flame retardancy and enables PC to achieve a UL-94 V0 rating at only 3%. In contrast, despite the dose similarities in the structures, DPZ with a direct P-N bond synthesized from DOPO and piperazine is much less effective. It requires 10% of DPZ for PC to gain a UL-94 V0 rating. On the basis of the study of diphenyl carbonate/flame retardants and the results of thermogravimetric analysis, Fourier transform infrared analysis, inductively coupled, plasma analysis, and morphology of PC/flame retardants, the base catalyzed decomposition of PC that leads to severe dripping is the key factor for the high efficiency of DFPZ.