The structure-property relationship of a series of polyoxo-metalates based ionic liquids (PILs) modified polypropylene (PP)/Intumescent flame retardant (IFR) composites were studied. The results showed that the chemical structure of PILs has a great effect on properties of PP composites. The flame retardancy of PP/IFR/PILs composites with a shorter alkyl group are better than that with a longer alkyl group. The PP/IFRIPIL composite with 0.5 wt% PIL with 1-butyl-3-methyl imidazole and 14.5% IFR gets the UL-94 V-0, but achieving the same UL-94 grade needs no less than 19.5 wt% IFR and 0.5 wt% PIL with 1-octyl-3-methyl imidazole. In addition, PILs modified by different groups have great effects on the mechanical properties, the PP/IFR/PIL composites containing 1-dodecyl-3-methyl imidazole obtain the best notch impact strength. The results show that the flame retardant and mechanical properties of PP/IFR/PILs composites can be tailored by changing the cation structure of PIL. (C) 2016 Elsevier B.V. All rights reserved.