We studied the preparation method of acicular magnesium pyroborate (Mg2B2O5), especially in relation to the prevention of formation of agglomerates, which generally form by heating mortar-ground mixture of MgO-H3BO3-KCl. Spray-drying of slurry, which contained reaction components and flux, gave a starting material with Mg and B in amorphous state and Mg and K almost uniformly distributed. The slurry was made by adding hot solution of H3BO3 and KCl to suspended solution of Mg (OH) 2 with 2 : 3 : 7 mole ratio of Mg : B : KCl. Heat treatment of the spray-dried mixture at 750°C gave small acicular crystals of Mg2B2O5 less than ca. 1μm in length without by-product such as MgO and MgB4O7. The product obtained by heating the mixture at 850°C for 3hrs contained only acicular Mg2B2O5 crystals ca. 5-15μm long and ca. 0.5μm thick, and did not contain agglomerates.