The influence of micrometeric zinc oxide (micro-ZnO) and nanostructured zinc oxide (nano-ZnO) on the flammability and thermal properties of acrylonitrile–butadiene–styrene (ABS)/poly(ethylene terephthalate) (PET)/ammonium polyphosphate (APP) systems was investigated. When 1 wt % micro-ZnO (or nano-ZnO) was incorporated into the ABS/PET/APP systems, the best flame-retardancy rating of V0 was achieved during UL94 tests, and the limiting oxygen index value was 31 (or 30). The thermogravimetric analysis indicated that the addition of zinc oxide (ZnO) improved the thermal stability and the yield of char residues of the ABS/PET/APP systems. Fourier transform infrared spectrophotometry showed that ZnO improved the efficiency of APP, and the char residues of the ABS/PET/APP + 2 wt % ZnO systems had higher phosphorus contents. Scanning electron microscopy showed that the surface of the char for ABS/PET/APP + 2 wt % micro-ZnO (or nano-ZnO) improved significantly with a cohesive and dense structure; however, that for ABS/PET/APP + 5 wt % micro-ZnO (or nano-ZnO) exhibited a looser char layer with bigger holes. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.