Physical properties of activated carbons (ACs) modified with various acids were investigated by infrared (IR) and N2 gas adsorption at 77K. Adsorption of NH3 over various acids modified ACs was carried out to determine the relationship between the adsorption capacity and physical properties of the modified AC. The BET specific surface area and pore volume were decreased with increasing the concentration of phosphoric acid for modification, but the average pore size was not almost changed. Among the acid modified ACs, H3PO4 modified AC (PN/AC) showed the best adsorption capacity. The amount of NH3 adsorbed was increased with H3PO1 contents for modification, but was suddenly decreased at H3PO4 content of above 20wt%. Relationship between the physical properties and amount of NH3 adsorbed was not clear, but it was thought that the adsorbed amount could be depended on functional group of surface.