Microstructure change and atomic disordering in MgO·nAl2O3 (n=1.1) irradiated with 350MeV Au ions (Se=35keV/nm) were investigated through transmission electron microscopy (TEM) and high angular resolution electron channeling X-ray spectroscopy (HARECXS) techniques. High resolution TEM revealed that each ion track maintains crystalline structure. The core region of ion track is found to reveal a lattice fringe with a half period of spinel matrix, suggesting the phase transformation from spinel to rock-salt structure. HARECXS analysis clearly showed progress of cation disorder at a significantly large region of ∼10nm in diameter. These results are compared with the previous results of 200MeV Xe ion irradiated spinel (Se=25keV/nm). The structure of ion tracks is found to consist of three concentric circle structures: the defective core region (∼2nm in diameter), strained region (∼5nm) and cation disordered region (10–12nm).