Recently a growing interest has arisen about radial line slot array (RLSA) antennas that appear to be simple and low-cost solutions for applications at high microwaves and millimeter waves. In this paper a computationally efficient method of moments (MoM) formulation that make use of an approximate asymptotic formula for the slot mutual admittance is presented. The formula accuracy to the asymptotic order 0(r-3/2) gives sufficient accuracy for moderate slot distance (1.5-2 wavelength). This permits, in a typical RLSA analysis, avoiding the numerical integration in more than 90% of the slot pairs with a save of CPU time of the same order.