Adsorption behavior of ammonium ion on both the crystalline Linde Type A zeolite with sodium ions (Na-LTA) and the noncrystalline pseudomorph after Na-LTA were investigated. Na-LTA transformed to the amorphous phase which preserves original morphology, that is, noncrystalline pseudomorph, by a treatment of acid solution. The pseudomorphic transformation of Na-LTA was occurred by 0.06 M HCl treatment and was confirmed by powder XRD method and SEM. The adsorption amount of ammonium ion on the noncrystallne pseudomorph was ca. 45% of that on the crystalline Na-LTA, but was comparable to that of natural zeolite such as mordemte and clinoptilolite. The linear relationship between the amount of adsorbed ammonium ion and the amount of dissolved sodium ion on the noncrystalline pseudomorph after Na-LTA was obtained. It indicated that the ammonium adsorption on the noncrystalline pseudomorph was occurred by the ion-exchange mechanism.