A new slot array antenna is presented which is based on triangular waveguide. The modes and the leakage of an imperfectly fabricated waveguide are examined by simulation, and this helps to identify a low cost structure. The slots are in the two side walls of the triangular waveguide. A 5-element, equally-excited antenna array is designed and simulated for 2.4GHz. The gain is 9.2 dBi and the −10dB impedance bandwidth is over 3%. The advantage of this array configuration is its simplicity, low cost and good performance.