The luminance and luminous flux of external-electrode, mercury-free, Xe flat discharge fluorescent lamp for LCD backlights and lighting equipment are equivalent to those of conventional Hg fluorescent lamps. However its efficacy is lower. In this paper, improvement of efficacy was attempted. First, the electrode width was reduced from 1.0mm to 0.5mm in order to widen the operating pressure margins. This enabled the Xe partial pressure to be increased from 1.2 kPa to 3.0 kPa, which increased efficacy 60%. Then a MgO, which has a high secondary electron emission coefficient, was coated on the dielectric layer, improving efficacy a further 10%. As a results, an efficacy of 35 lm/W with a luminance 10,100 cd/m² were obtained with a 2.8-inch diagonal lamp.