Abstract: The effect of La(OH)3 on the flame retardancy and the mechanical properties of intumescent flame retardantcontaining polypropylene composites was studied. The limiting oxygen index, UL94 test, thermogravimetric analysis, scanning electron microscopy, tensile strength, elongation at break, and impact strength were employed to investigate the synergistic effects of La(OH)3. It showed that La(OH)3 could enhance the limiting oxygen index of the composites and made the retardancy reaching UL94 V0. The limiting oxygen index of the composites with 0.5 % La(OH)3 achieved 33.0 %. And La(OH)3 contributed to the formation of a compact char layer on the surface of the composites and increased the amount of the char residue. It could be speculated that the La(OH)3 reacted with intumescent flame retardant and caused the crosslinking of the composites, which increased the viscosity and resulted in a more homogenous and compact structure of char. And a suitable amount of La(OH)3 could also benefit the mechanical properties of the composites.