A numerical study was performed to find the best position for error sensors and secondary sources of an active noise barrier. In this paper, a numerical study has been done, using MacDonald's solution, to calculate diffracted sound pressures of barrier's edges. In addition to the barrier's top edge diffraction, the vertical edges diffractions considered too, so this study is independent of barrier lenght. In order to consider the effect of ground, image theory is used. In order to have the maximum attenuation in shadow zone, one needs to know the best position for the error sensors and secondary sources as well as their numbers. This paper considered different positions for the error sensors and secondary sources and the best arrangement is determined in the results. In addition, according to the lenght of barrier, the optimum number of error sensors and secondary sources defined.