Ammonia tempetature-programmed desorption (TPD) with water vapor treatment method was applied to MFI-type ferrisilicate, and theoretical analysis was carried out to determine the acid amount and strength exactly. At the low iron content when isomorphous substitution of iron into the silicate matrix was predominant, a quite strong acid site with 150-180 kJ mol(-1) adsorption heat of ammonia, which was stronger than that on aluminosilicate, was observed. The determined acidic property was in good agreement with microcalorimetry. Therefore, it is concluded that the isomorphous substitution of iron into the MFI structure generates a stronger acid site than that by aluminum.