The present invention relates to a production of 1,3-butadiene and methacrolein simultaneously by catalytic oxidation of mixed butenes composed of butene-1, cis-2-butene, trans-2-butene (unless otherwise mentioned, these three butenes will referred to as n-butenes hereinafter) and iso-butylene more particularly, this invention relates to a process for preparing 1, 3-butadiene and methacrolein simultaneously in high yields by oxidizing the mixed butenes mainly composed of n-butenes and isobutylene with molecular oxygen at an elevated temperature in the presence of a catalyst having the empirical formula:Ni.sub.a Co.sub.b Fe.sub.c Bi.sub. d L.sub.e M.sub.h Mo.sub.f O.sub.gWherein Ni represents nickel, Co represents cobalt, Fe represents iron, Bi represents bismuth, Mo represents molybdenum, O represents oxygen, L represents at least one element selected from the group of phosphorus, arsenic and boron, M represents at least one element selected from the group of potassium, rubidium and cesium, and a,b,c,d,e,f,g and h represent numbers of atoms of Ni, Co, Fe, Bi, L, Mo, O and M, respectively, with the proviso that when (f) is 12, (a) and (b) independently are numbers of 0 - 15, (a)+(b) being 15 - 1, (c) is a number of 7 - 0.5, (d) is a number of 4 - 0.1, (g) is a number of 36 - 98 which is accounted from typical oxides of these metals, (e) is a number of 0 - 4 and (h) is a number of 0.01 - 1.0. According to the present invention, the final products, i.e. 1.3 - butadiene and methacrolein are produced with a high selectivety from said mixed butenes without excessive oxidation of the olefins to undesired higher oxidation products of carbon such as carbon monoxide, carbon dioxide, maleic acid and acetic acid.