The synthesis and thermal decomposition behavior of the phenylalanine/M²⁺-Al layered double hydroxides (Phe/Mg-Al, Mn-Al and Zn-Al LDHs) have been investigated. The Phe/LDH was easily synthesized by the conventional coprecipitation method. The basal spacing of the Phe/LDHs was expanded to 0.86 (Mg-Al), 1.80 (Mn-Al) and 1.86 nm (Zn-Al), respectively, which supported that Phe was intercalated into interlayer space of the LDH as an anion or an amphoteric ion form. The thermal decomposition of the intercalated Phe was influenced by the kind of a divalent metal ion in the LDH basal layer. In particular, the Mn-Al LDH was found to have the strong catalytic action for the decomposition of the intercalated Phe. From the FT-IR, HPLC and NMR measurements, it was proved that the intercalated Phe was decomposed via the formation of N-benzoylbenzamide (NBBA) regardless of the kind of LDHs, which compound never formed in the thermal treatment of Phe itself. Therefore, it is expected that the two dimensional LDH interlayer space can be used as a novel nano-scale reactor for the synthesis or the decomposition reaction of organic compounds.