In this paper modification potential of dispersive clay by white natural alum is evaluated. The studied clay is taken from basin of a constructing dam, called “Mirzakhanloo”, located in Zanjan province, northwest of Iran. Dispersion potential of soil and its modifiability were evaluated using pinhole, single and double hydrometric, chemistry and standard compaction tests. The influence of alum on the Atterberg limits of soil was also studied. Tests results indicated that adding the alum and curing time up to 28 days, under hydraulic gradients up to 26.84, decreases the dispersivity of the modified soil. Increasing the alum content increases the plasticity limit. Liquid limit, plasticity index and pH of the soil were increased by adding the alum up to 0.6%, and then decreased. Both the plasticity and liquid limits were decreased by curing time, however the plasticity index variations were limited. Electrical conductivity (EC), and Sodium absorption ratio (SAR) of the soil were increased and total suspended solids (TSS) in the leached water were decreased by increasing the alum content and curing time. Generally, addition of 3% alum stabilized the used soil knowing that maximum dry density and optimum moisture content were obtained at 1% of alum.