Epitaxial thin films of ordered double perovskite La2NiMnO6 have been studied by a combination of high-resolution scanning transmission electron microscopy, quantitative electron energy loss spectroscopy, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD) spectroscopy. Our results show the nominal oxidation states of Ni and Mn ions to be Ni2+ and Mn4+ thus the ferromagnetism in ground state is mainly due to Ni2+-O-Mn4+ superexchange interactions. In addition, short-range ferromagnetic correlations are observed above the Curie temperature (TC˜280K) from XMCD measurement, which are likely induced by antisite defects against long-range ordering of the Ni/Mn sublattice. The XMCD results also demonstrate that the Ni2+ and Mn4+ ions are ferromagnetically aligned but exhibit large differences in the spin and orbital contributions to their effective magnetic moments.