Modified magnesium hydroxide (Mg(OH)2) particles as flame retardant were prepared by one step method using magnesium chloride hexahydrate (MgCl2·6H2O) and sodium hydroxide as raw materials and stearic acid as modification agent. The influences of reaction temperature, reaction time and sedimentation time on activation index and oil absorption ratio were explored. They were characterized by means of SEM， XRD， LPSA and TGA, and applied initially in polypropylene (PP). The results showed that the best technological conditions were: reaction temperature 80 ℃, reaction time 90 min, sedimentation time 90 min. The obtained magnesium hydroxide particles showed good dispersibility and compatibility with PP. The PP sample with 50% magnesium hydroxide showed excellent flame retardant property with the limited oxygen index (LOI) of 28.5 and UL94 retardant degree of V 0.