A first catalyst for reducing nitrogen oxides comprising a crystalline silicate containing an iron in β-framework structure wherein a SiO2/Fe2O3 mol ratio is 20-300 and at least 80% of the contained iron is an isolated iron ion Fe3+. A second catalyst for reducing nitrogen oxides comprising a crystalline silicate containing an iron in β-framework structure wherein a SiO2/Fe2O3 mol ratio is 20-300 and log(SiO2/Al2O3) by mol is at least 2. A predominant part of the contained iron is isolated iron ion Fe3+ and at least a part thereof preferably has a tetrahedral coordination. These catalysts have high hydrothermal stability and exhibit enhanced activity for reducing nitrogen oxides by a reaction with a reducing agent such as ammonia, urea or an organic amine in a broad temperature range between lower temperature and higher temperature.