Abstract: Mg-Al layered double hydroxides (LDHs) precursor was synthesized by coprecipitation method under mild conditions. The crystallization of LDHs was modulated with a small amount of ammonium polyphosphate (APP) to prepare APP-LDHs and the influence of APP at different mass fraction on the crystallization was investigated. When the addition of APP in LDHs suspension was 0.8wt%, the APP-LDHs was mixed with pentaerythritol (PER) and silane coupling agent KH-550 via ball milling and the modified LDHs (modified-LDHs) was completed ultimately. XRD, FTIR, SEM, TG were utilized to characterize the structure, morphology and thermal stability of the LDHs. The fire retardancy of LDHs/PP composites were evaluated by UL-94 test and limiting oxygen index (LOI), as well as the mechanical behavior was assessed, using flexural strength and notch impact strength. The results demonstrate that the crystal growth of LDHs is inhibited along the stacking direction in the presence of APP, but the grain size which represents layer growth remains unchanged. SEM images of the LDHs indicates sheet-like structure of the as-prepared LDHs, while the APP-LDHs modified by PER and KH-550 via ball milling shows that the modified-LDHs formed a regular powder with particle size of 100 nm to 250 nm. The modified-LDHs performs a better thermal stability at higher temperature. PP fills with 20 % mass fraction of LDHs burnt without droplet and the surface of LDHs/PP composites residue are covered with an expanded char network. Modified-LDHs/PP composite has superior flexural strength and notch impact strength compares with LDHs/PP and APP-LDHs/PP composites.