In this study, mixing petroleum-based low-density polyethylene (LDPE) with biodegradable poly(lactic acid) (PLA) is used as polymer matrix. Boron compounds, metal hydroxides, melamine (MLM), ammonium polyphosphate (APP), and pentaerythritol (PER) were used as reinforcement materials to improve flame resistance of polymer matrix. The composite materials were characterized by Fourier transform infrared spectroscopy, limiting oxygen index (LOI), thermogravimetric analysis, mechanical test, and scanning electron microscopy analyses. The LOI analysis showed that for samples, which included MLM, APP, and PER, the LOI values were dramatically improved. Especially, the LOI value of sample Q (LDPE80/PLA20/APP30/PER15/MLM15/ZB3) was enhanced about 95.17% compared to polymer mixing. (c) 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48960.