Expander production is including three main processes as Grinding, mixing and Sorting that grinding process is for making a uniform mixture from different particle sizes of the components.So the final mixture optimum particle size is an important parameter on expander and affectingly NAM performance.In this study we investigate the expander particle size effect on the performance of the NAM of LAB to find out the optimum expander particle size for SLI application.We used a prototype ball mill to prepare expanders with different particle sizes.To evaluate different produced expanders,a techniques based on cyclic voltammetry is used.The electrolyte was prepared by dissolving prepared expander with the amount of 3gr in 100cc H2so4 5M then a lead electrode immersed in the H2so4 solution containing the expander is cycled within the potential range of -0.325 TO -0.725 V.In Pb│PbSO4 electrode, the passivation of surface is prevented by expander that leads to increasing in the current passed through the negative plate resulting in an increased area under the i-t curve.The observed results from cyclic voltammetry test shows that decreasing expander material particle size will increase Imax and discharge process is more effectively occurred.In PbSO4│Pb electrode,expander impedes the process of PbSO4 reduction,so they normally decrease the cathodic Imax when compared to no-expander condition and charge acceptance is negatively affected. Good expander will decrease this effect and the observed results from cyclic voltammetry test shows that higher current can be achieved during charge process by decreasing expander material particle size that will cause to better charge acceptance.