A magnesium hydroxide particle having a BET specific surface area of 80 m.sup.2/g or more and a manufacturing method thereof. The magnesium hydroxide particle has a BET specific surface area of 80 to 400 m.sup.2/g and is represented by the following formula (1): Mg(OH).sub.2-x(CO.sub.3).sub.0.5x.mH.sub.2O (1) wherein x and m satisfy the following conditions: 0.02.ltoreq.x.ltoreq.0.7. 0.ltoreq.m.ltoreq.1.