The present invention relates to a process for preparing acrylic acid by oxidizing acrolein and to an oxidation catalyst.More particularly, the present invention relates to a catalyst and a process for preparing acrylic acid by oxidizing acrolein or a mixed acrolein gas obtained by vapor phase oxidation of propylene with molecular oxygen in the presence of an oxidation catalyst of the empirical formula:Mo.sub.a V.sub.b T.sub.c A.sub.d O.sub.ewherein Mo, V and O represent molybdenum, vanadium and oxygen, respectively, T represents tungsten or antimony, A represents an alkali metal, and a, b, c, d and e represent number of atoms of Mo, V, T, A and O, respectively, and when a is 12, b is 0.5 to 6, c is 0 to 6, d is 0.01 to 1.5, preferably, 0.15 to 1.2 and e is naturally determined by the valence requirements of the other elements present.