When an ignition system operates, conducted interference caused by transient voltage of the primary circuit will impact on the accumulator and the disturbance will travel throughout the vehicle via wiring. The ignition system can disrupt the Electronic Control Unit and the normal operation of other electrical devices. Spark plug gap noise simultaneously produces very strong radiation interference in a vehicle. We analyzed the changes of the ignition coil voltage and current when an ignition switch was turned on and off. A discharged circuit model of spark plug also was set up. The electric field distribution in an automobile was calculated. For these calculations, the highvoltage wire was considered to be a unipole antenna. This simulation indicates that using either damped wire or adding spark plug resistance can suppress spark noise effectively.