Abstract: Nano-sized magnesium hydroxide particles (MH) were modified with vinyl silane followed by in situ polymerization of styrene. The MH/polypropylene (PP) composites were prepared by melt compounding of PP with MH and modified MH (MMH)，respectively. The morphological structure and rheological properties were characterized by scanning electron microscopy and capillary rheometer. The results show that the MH/PP composite is a shear-thinning non-Newtonian，and its apparent viscosity and non-Newtonian index (n) are bigger than those of the PP melt. With increasing the MH content，its apparent viscosity and n increase. The low MH content decreases the flow activation energy (ΔE) of the MH/PP composites melts，but ΔE values of the composites increase with MH content. However，the in situ polymerization of styrene on the surface of MH enhances the dispersion of MH in composites. The “ball bearing” effect of the modified MH particles decreases the apparent viscosity of MMH/PP composites，their n and ΔE values are lower than those of PP，and ΔE values decrease with increasing the MMH content in composites.