One of the common methods in the study of cosmic rays induced EXTENSIVE AIR SHOWER on the Earth's surface is using an array of scintillation detectors. Dimensions and distances between detectors are among the most important parameters that determine the energy range detectable by the array. Furthermore, the geometry of the array has an important role in determining of optimum distance Ropt, which is defined as the distance from the SHOWER core, where uncertainty in lateral density of secondary particles is minimized. In the present work, dependence of Ropt on array geometry and characteristics of cosmic rays has been studied for the Alborz-1 array. It is shown that the value of ropt is independent on SHOWER characteristics like energy, incident angle and primary mass. On the other hand, according to our results, the ropt value has explicit dependence on array geometry specially distances between detectors. Our investigation shows that the cluster layout is the best layout for the detectable energy range of ALBORZ-1 array (1015 eV). We have found Ropt= 9 ± 1m for the ALBORZ-1 array.