Directly starting from d,l-lactic acid (LA) and pyrimidine-2,4,5,6-tetramine (PTA), the copolymer P(LA-co-PTA) as a novel potential solid compatible polymeric flame retardant is synthesized as designed via melt polycondensation. When the molar feed ratio LA/PTA is 60/1, the optimal synthetic conditions are discussed. After the prepolymerization at 140°C for 8 h, using 0.5 wt % stannous oxide as the catalyst, the melt copolymerization at 160°C for 4 h gives the copolymer with the biggest intrinsic viscosity 0.88 dL g−1. The structures and properties of P(LA-co-PTA)s at different molar feed ratios are characterized by FT-IR, 1H-NMR, 13C-NMR, GPC, XRD, DSC, and TGA. The decomposition temperatures of P(LA-co-PTA)s are higher than these of homopolymer poly(d,l-lactic acid) (PDLLA). All copolymers have higher char yield than PDLLA, and the more PTA in the feed content, the higher char yield. What's more, there are some residues at 700–800°C, indicating that P(LA-co-PTA)s have good charring ability. When the monomer PTA is introduced into polylactic acid by chemical bonding as purine (PU) unit formed during the condensation, both the PTA's relatively higher nitrogen content and the PU's similar structure with flame retardant benzimidazole are beneficial to improve the thermal stability and charring ability, especially the latter. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40275.