As　a　heavy　metal,　Pb　is　one　component　in　coal-fired　flue　gas　and　is　widely　considered　to　have　a　strong　negative　effect　on　catalyst　activity　in　the　selective　catalytic　reduction　of　NOx　by　NH3　(NH3-SCR).　In　this　paper,　we　investigated　the　deactivation　mechanism　of　the　Mo-Ce/Zr-PILC　catalyst　induced　by　Pb　in　detail.　We　found　that　NO　conversion　over　the　3Mo4Ce/Zr-PILC　catalyst　decreased　greatly　after　the　addition　of　Pb.　The　more　severe　deactivation　induced　by　Pb　was　attributed　to　low　surface　area,　lower　amounts　of　chemisorbed　oxygen　species　and　surface　Ce3+,　and　lower　redox　ability　and　surface　acidity　(especially　a　low　number　of　Bronsted　acid　sites).　Furthermore,　the　addition　of　Pb　inhibited　the　formation　of　highly　active　intermediate　nitrate　species　generated　on　the　surface　of　the　catalyst,　hence　decreasing　the　NH3-SCR　activity.