In a typical deep brain stimulation (DBS) process, doctors depend on the patient's response in tuning the stimulation parameters, which is time consuming. In this process, the volume of tissue activated (VTA) is an important index to describe the stimulated region for helping adjusting suitable stimulation parameters. However, the VTA is hard to measure in clinical trials. The reason is because the brain tissue along the contact of the electrode varies with time since the electrode is implanted chronically. Some previous studies have shown that electrode impedance value significantly affects the size and shape of VTA. However, there is not a precise mathematical model to further investigate the relationship between electrode impedance and VTA. This study thus uses the technique of finite element analysis (FEA) for electromagnetic field simulation, in order to quantify the impact of clinically relevant impedance variability on the VTA during DBS. Through the quantitative description, the VTA can be predicted. Doctors can then tune the stimulation parameters efficiently.