This paper compares two methods for synthesizing a carbonate compound, Mg-Al-CO3 (Mg:Al=2:1), which is a layered double hydroxides (LDHs). The methods are mechanochemical method and hydrothermal method. The products characterized by XRD、SEM、FT-IR and TG-DTA. The results indicated that the target material was synthesized by different methods. The two kinds of well-crystallized materials with the same basal spacing d003=0.77nm, but their morphology were not same. The crystallite size of hydrothermal was larger than mechanochemistry.