A quantum dot (QD) can capture and emit carriers, the behavior of which is similar to that of a giant trap, and stacked QDs with a size-controlled growth show a strong tendency to align vertically. The discrete energy level properties of the self-assembled vertically stacked InAs QDs in Al0.5Ga0.5As are studied by means of deep level transient spectroscopy (DLTS) and photoluminescence (PL) spectroscopy. The DLTS measurement displays the spectra of hole and electron discrete energy levels as positive and negative peaks, respectively, which illustrates that the DLTS is a capable tool to study the optical and electrical properties of the QDs. The PL emission peaks are found to completely correspond to the DLTS signals.