In this study, negative-polarity, standard lightning impulse voltages with different magnitudes were employed throughout the experiments. The percentage of SF6 in Ar was varied from 1% to 90% and from 1% to 75% at a total gas pressure of 50 kPa and 100 kPa, respectively. The breakdown voltage of each gas mixture was recorded at 50% breakdown probability. Similar to earlier studies under static electric fields it is observed that the breakdown voltages of Ar+SF6 has increased dramatically when mixed with a small percentage of SF6 under impulse voltages. However, the breakdown voltages were found to be higher than positive polarity, non-uniform and uniform fields.