Macromolecule flame retardant melamine-dicyandiamide-formaldehyde-phosphoric acid (denoted as MDFP) was used as the shell material to synthesize zinc hydroxystannate@MDFP (denoted as ZHS@MDFP), a novel composite flame retardant with core-shell structure, via masterbatch method. The morphology and structure of ZHS@MDFP were analyzed by means of transmission electron microscopy, X-ray powder diffraction, Fourier transform infrared spectrometry, and thermal analysis. Moreover, the effect of ZHS@MDFP as a flame retardant on the flame-retardant behavior and mechanical properties of flexible poly(vinyl chloride) (denoted as PVC) was investigated. It has been found that as-synthesized ZHS@MDFP composite flame retardant has core-shell structure. Besides, as-synthesized ZHS@MDFP as a core-shell flame retardant is superior to ZHS in increasing the limiting oxygen index and decreasing the smoke density rating of PVC, which is because the decomposition of MDFP shell as the blowing agent expands the char layer thereby improving the flame-retarding capability of ZHS core. More importantly, ZHS@MDFP does not cause damage to the tensile strength and elongation at break of PVC matrix, which implies that the MDFP shell favors to improve the compatibility between ZHS and flexible PVC matrix. (C) 2013 Society of Plastics Engineers