We have studied C(2)H(4) and O(2) molecules separately or simultaneously for adsorption on V(n) (n = 2-8) clusters, and V(n) clusters catalyzed ethylene oxidation to acetaldehyde using spin-polarized density functional theory calculations. Molecular adsorption and clear size-dependent adsorption energy are predicted for C(2)H(4). O(2) is dissociately adsorbed with nearly constant adsorption energy. In the case of coadsorption, O(2) and C(2)H(4) adsorb on the V(n) surface simultaneously. Each keeps the same adsorption form, molecular or dissociative, as in separate adsorption. The noted cooperative effect is noted in C(2)H(4) and O(2) coadsorption, which activates the C-C double bond of C(2)H(4) and favors its oxidization. Furthermore, both the separate and coadsorptions result in magnetic enhancement or reduction of V(n), which is found to be dependent on the cluster size and the adsorbates. In addition, we reveal the reaction mechanism of V(2) (V(6))-catalyzed ethylene oxidation to acetaldehyde and find the overall reaction is exothermic and barrierless.