Electro-Chemical Discharge Machining (ECDM) is an effective electrical machining method for nonconductive materials such as a glass. However, precise control of the spark generations in ECDM process is difficult because of the complexity in hydrogen dielectric film formation. The spark generation in ECDM process is closely related with the anode effect phenomenon which has been explained as a consequence of altered wettability of the electrode surface during electrolysis process. The initiation of the anode effect is mainly influenced by critical current density which is dependent on number of parameters such as the wettability of the hydrogen bubble, the surface conditions of electrode, and hydrodynamic characteristics of the bubbles. In this study partially side insulated electrode which maintains the constant contact surface area with electrolyte has been applied to the ECDM process in order to manage regular critical current density. The experiment results demonstrated that ECDM with side insulated electrode immerged in the electrolyte generates more stable spark discharge compared to non-insulated electrode, and the machining process repeatability was improved. Experiments were conducted to investigate the effects of the side insulation to the process quality.