It was found that carbon chain length of fatty acids strongly affects polymorphic selection in the cooling crystallization of glycine from water/saturated fatty acid emulsions. Two-dimensional packing density of saturated fatty acid head groups, which is inversely proportional to the number of carbon atoms, was shown to be responsible for polymorphic selection of glycine: γ-glycine was obtained from the emulsions of hexanoic acid and octanoic acid, whereas α-glycine was found to crystallize from the emulsions of dodecanoic acid, tetradecanoic acid, hexadecanoic acid and octadecanoic acid. Those results indicate that molecular structure of γ-glycine is only well matched with molecular structure of head groups of hexanoic acid and octanoic acid at the interface of the emulsion, and thus such molecular interface provides the preferential site for the organization of γ-form crystal structure from the liquid-like cluster of glycine.