In this paper, the authors propose a leaky wave slot antenna array for azimuthally omnidirectional coverage. Slot elements were arranged in cascade and series-fed by a CoPlanar Waveguide (CPW). The most important novelty of this paper is that the whole array, including all the radiating elements and feeding structures, was arranged on a single metal layer. This simple structure has the merits of easy fabrication and low cost, especially at higher frequencies, such as millimeter wave band. Moreover, the proposed antenna array was folded around a center-hollowed columnar substrate to achieve omnidirectional radiation pattern in the azimuthal plane, with the gain variation less than 1.1dB, which is similar to previous omnidirectional antenna array.