The formation of rectangular plate-like alpha-MnOOH and sheet-like gamma-MnOOH crystals has been achieved under ambient conditions. A gradual crystallization of manganese compounds with the diffusion of ammonia vapor inhibits random oxidization of divalent manganese ions and the formation of aggregates of Mn3O4 crystals. The simple method using ammonia vapor is applicable to the development of manganese-based compounds with ordered structures.