Abstract: Disclosed is a method of preparing a dicyclic phosphorus-melamine compound and a flame retardant for use with a polymer using the same, in which melamine or a melamine derivative is incorporated into a dicyclic phosphorus compound via substitution, whereby the dicyclic phosphorus-melamine compound for actualizing a non-halogen flame retardant is obtained and is advantageous due to its water-insolubility and enhanced impact strenght and tensile strength after treatment due to increased dispersibility, thus having various applications. The method consists of reacting pentaerythritol with phosphorus oxychloride, to synthesize pentaerythritol ester of phosphoro chloridic acid, which is then dissolved in water, to substitute the OH group for the CI group in the ester, followed by the substituted ester with melamine or melamine derivative, to prepare the dicyclic phosphorus-melamine compound.