Alumina is a promising candidate for fabricating the gate of diamond metal oxide semiconductor field effect transistor (MOSFET) due to its outstanding nominal properties: A high gap of 8.8 eV and a high static dielectric constant of 9. However, such properties are strongly dependent on the synthesis. As gate oxides are usually very thin layers (5-50 nm), investigating its properties is not straightforward. Electron energy loss spectroscopy in scanning transmission electron microscopy (STEM-EELS) methodology is reported in the nm-scale range. Monochromatic 60 keV electron beam is used to obtain the low energy loss spectrum in order to allow an accurate zero loss peak deconvolution and to avoid Cherenkov effect. The low energy loss spectrum is used to extract the bandgap along diamond-alumina interface and to perform Kramers-Kronig analysis to obtain the complex dielectric function of the Al2O3. High resolution electron microscopy (HREM) and STEM-EELS investigations show that the oxide phase of our sample is. alumina. Its measured bandgap is 6.8 eV and the dielectric functions yield a value of 3 for the high frequency dielectric constant.