The objective of this study was to know the effect of particle size of the laterite contained in reactors filled with laterite and sand on the removal of the physicochemical parameters of a well water. Three reactors filled from the bottom to the top with 10 cm of granitic gravel, 50 cm of sand, 10 cm of standing water and 30 cm of different particles sizes of laterite (granular, very coarse and medium) were used. The depths of the sand and the standing water were fixed according to a previous study. pH, conductivity, turbidity, suspended solids, ammonium, nitrate, lead and total iron were analyzed according to standard norms. The results showed that pH increased with decreasing particle size while conductivity decreased. With regard to suspended solids and turbidity, the removal was (98.9%) and of the same order in all the reactors. Removal efficiency of ammonium ranged from 97.67% to 98.1% in the reactors. The reactors also removed nitrate, total iron and lead. In addition, the concentrations of nitrate, total iron and lead decreased with increasing particle size. However, the reactor filled with medium laterite and sand was the most efficient.