Abstract: A process for producing methacrylic acid and/or methacrylic acid ester by oxidative dehydrogenation of isobutyric acid and/or isobutyric acid ester, wherein the improvement comprises using a catalyst having the composition represented by(Mo).sub.a (V).sub.b (P).sub.c (Cu).sub.d (As).sub.e (X).sub.f (O).sub.g(where X denotes at least one element selected from the group consisting of lithium, beryllium, magnesium, barium, tin, lead, chromium, tungsten, and tellurium; subscripts a, b, c, d, e, f, and g each denote the ratio of the respective elements; b, c, d, e, and f are in the range of 0.ltoreq.b.ltoreq.3, 0.2.ltoreq.c.ltoreq.2, 0.005.ltoreq.d.ltoreq.2, 0.005.ltoreq.e.ltoreq.0.2, and 0.005.ltoreq.f.ltoreq.2, respectively, when a is 12, and the value of g is determined according to the ratio and valence of the other elements), and at least a part of the composition having the heteropoly acid structure.