Various foreign ions were added as promoters into iron phosphate catalysts and their effects on performance in oxidative dehydrogenation of isobutyric acid to methacrylic acid were studied. The best performance was obtained with Pb2+ promoters and the optimum P/Pb/Fe atomic ratio was found to be in the range of 1.2-1.3/0.15/1 to 1.3-1.4/0.3/1. Properties and catalytic performance of an iron-lead binary phosphate with a P/Pb/Fe atomic ratio of 1.4/0.3/1 were compared with those of an iron phosphate with a P/Fe atomic ratio of 1.2. It was concluded that the Pb2+ promoter which exists in the catalyst in the form of Pb2P2O7, plays a role in suppressing the consecutive oxidation of the produced methacrylic acid to CO,. The effects of the Pb2+ promoter were also discussed on the basis of the data of X-ray diffraction spectra, surface area, acidic properties, and reducibility.