The limitation of the pulse repetition frequency (PRF) of an airborne synthetic aperture rader (SAR) system is not a serious problem to obtain high azimuth resolution and wide swath imaging compared with a spaceborne SAR system. Hence, continuous high azimuth resolution imagery over a wide area can be obtained using an antenna having a wide beamwidth. Since a small antenna with a large beamwidth has very low gain, which results in difficulty in detection; the azimuth beam pattern optimization of a large active phase array antenna is needed for airborne SAR system optimization. To improve the airborne SAR system performance, such as the noise-equivalent sigma zero (NE sigma(0)), the azimuth resolution, the radiometric accuracy (RA), and the azimuth ambiguity ratio (AAR), we prersent an optimal azimuth beam pattern mask template and suggest an azimuth beam pattern satisfying the mask template using the particle swarm optimization (PSO). The mode having the proposed beam pattern guarantees continuous and high resolution images, simulataneously. Using a point target simulation, the advantages of the mode are shown compared to strip-map and spotlight modes.