A novel thiourea-intercalated kaolinite flame retardant (K-TU) was prepared by intercalating thiourea (TU) into kaolinite. The structure of K-TU was characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA), and X-ray diffraction. K-TU in association with intumescent flame retardants (IFR) was then introduced into polypropylene (PP) by melt blending. The flammability and thermostability were evaluated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimetry (Cone), and TGA. The results showed that the sample PP/23.5 wt% IFR/1.5 wt% K-TU passed UL-94 V-0 level, had a LOI of 35.4 %, and its peak heat release rate was reduced to 291 kW/m(2) from 1474 kW/m(2) of neat PP. The TGA results demonstrated that K-TU increased the thermal degradation onset temperature and enhanced the char formation of PP/IFR composites. Meanwhile, the mechanical performance of PP/IFR composite was improved by the presence of K-TU. It was proposed that the improved flame retardancy of PP composite was ascribed to both the shielding effect formed by exfoliated K-TU molecules and gas phase effect caused by inert gases mixture released from TU, which could prohibit the heat and oxygen transfer and dilute the concentration of combustible gases during burning.