The effects of the geometry and polarity of electrodes on the ionization current during combustion were experimentally investigated using compressed natural gas (CNG) as a fuel in a constant volume combustion bomb. Spherically, expanding flames are measured using the schlieren photography technique to establish the relationship between the initial flame kernel development and the waveform of ionization current. The study shows that the parameters, including the polarity and the size of electrodes, have little impact on ionization current waveform. The information on combustion in ionization current, such as ignition, front flame, and post flame, are fully observed and clearly identified. Another aspect of this study is to attach a disk either at the anode, cathode, or both electrodes to increase the surface area of the disk or to change the location of the disk. The experiments shows that the first peak position of ionization current is mainly related to the flame kernel development at which stage the ions and electrons are produced by chemi-ionization processes, while the location and area of the disk have a slight influence on its peak position. Large amounts of ions and electrons during both chemi-ionization and thermal ionization are Collected if the surface area of electrode is large, and more important is the anode area. It is also found that the peak values of both front flame and post flame as well as their integral values reach the highest values when the distance of the front flame kernel to the electrode is 5 mm, which is considered as the optimum distance for detecting the ionization current during combustion. However, the electrode parameters have an insignificant effect on the interval from the ignition to the second peak and the durations of front flame and post flame.