The effect of UV preionization by a KrF laser on CO2‐laser‐induced optical breakdown is studied experimentally. The electron number density as well as the breakdown probability is measured as a function of KrF‐laser energy. The results show that the KrF‐laser preionization is quite effective in initiating the optical breakdown. Once a breakdown is initiated, however, the number of produced electrons is found to be independent of the KrF‐laser energy.