Magnesium hydroxide having a high aspect ratio, a production method thereof and a resin composition comprising the same.The method of producing the magnesium hydroxide having a long diameter (width) of not less than 0.5 μm and aspect ratio of not less than 10, comprising the steps of: (A) adding an alkali to and coprecipitating it with a mixed aqueous solution of a water-soluble magnesium salt and a monovalent organic acid or a salt thereof, or (B) adding an alkali aqueous solution to and coprecipitating it with an aqueous solution of a water-soluble magnesium salt and adding a monovalent organic acid or a salt thereof to the resulting product; and (C) hydrothermally treating the obtained slurry at 100° C. or higher.