The urea decomposition property at high temperature has been used to control the pH value in the synthesis of layer compounds. The hydrotalcites of Mg-Al and Ni-Al with high crystallinity were synthesized by using this property. The urea decomposition property at high temperature has been used to control the pH value in the synthesis of layer compounds. The hydrotalcites of Mg-Al and Ni-Al with high crystallinity were synthesized by using this property.