A mixture comprising (i) an oxide, hydroxide or oxyhydroxide of aluminum, (ii) an aluminum-containing sulfate, (iii) an oxide or oxyacid of boron or an alkali metal salt thereof and (iv) an alkali metal sulfate is heated at a temperature of 900 to 1200° C to grow an aluminum borate whisker. In this process, the reaction is advanced apparently in the solid phase, and this process is advantageous over the conventional processes in that the yield is high and the isolation and purification of the reaction product is facilitated.