Quantum dots (QDs), which capture and emit carriers like a giant trap, are studied using deep level transient spectroscopy (DLTS). The electrons and holes in the QDs are emitted from the relevant energy levels to the conduction and valence bands, respectively, of the barrier layers with increasing temperature. The thermal emission energies from the QDs are related to their initial energy levels. In this paper, five-period vertically stacked InAs QDs in the barrier layers of a field-effect type structure are measured. The results agree well with capacitance-voltage and photoluminescence measurements. In addition, the dependence of DLTS signal on the pulse voltage and light illumination is presented. The results prove that DLTS is a powerful tool for the study of the electronic structure of QDs.