A process for the preparation of methacrolein by the vapor phase oxidation of isobutylene which comprises contacting isobutylene and oxygen with a catalyst composition comprising a catalyst system of the formula: MoaBibFecNidTleXfPgOh wherein X is one or more of Mn, Mg and Co and a, b, c, d, e, f, g and h represent respectively the number of atoms, wherein a is 12, b is 0.1 to 5, c is 0.1 to 5, d is 0.1 to 12, e is 1 or less but not O, f is 0 to 12, g is 0 to 5 and h is from 36 to 89. By such process, methacrolein is produced in a high selectivity and an excellent yield per each pass even at a large space velocity.