The effect of SF6 addition to a high-pressure nitrogen laser is studied. Stable UV laser action was achieved over a wide range of pressures to 4 atm with pure nitrogen, and a power increase by a factor of 2 was obtained by adding SF6 in a high-pressure region. When the partial pressures of N2 and SF6 were 100 torr, an output power of 173 kW was obtained. This was two times larger than for 100 torr pure N2 and 1.5 larger than the maximum output yielded with pure N2. As the N2 pressure increased, the optimum SF6/N2 mixing ratio decreased, and for N2 pressures over 3 atm, SF6 addition did not cause an increase in output.