Deep narrow slot (DNS) structures can be used in heat-dissipating fins of die-casting molds, grooves in flexible joints of gyros, and slots for inserts in tire molds. However, debris accumulation in the gap makes EDM process unstable and time-consuming, resulting in that slots with high aspect ratio are difficult to achieve. Various methods, such as flushing and electrode jumping, have been used to help evacuate debris and have achieved good results. Nevertheless, these methods treat debris with no distinction under the fact that some debris can naturally leave the gap with bubbles and fluid while the others cannot. This study aims to find out the difference between the easy-to-leave debris and the difficult-to-leave debris from the perspective of the particle size distribution of debris. Factors including currents, machining depths and electrode thicknesses, are studied on their influences on the particle size distribution of debris. In this paper, a laser particle analyzer (LPA) is used to obtain the statistical particle size distribution of debris, while a scanning electronic microscopy is used to observe and record the particle morphology and measure the particle diameters of partial debris. The knowledge of the difference in debris may help seek targeted ways to expel easy-to-leave debris and difficult-to-leave debris.