Abstarct: One of the most important geotechnical issues is problematic soils (dispersive soils, expansive soils, Collapsible soils, liquefying soils, soluble soils) which may affect the constructing projects on these soils and even may cause their destruction. In this research, collapsing soils are studied in particular. Collapsible soils in natural manner with low moisture content, can withstand relatively high pressure but after absorbing moisture and saturation, the gap between the soil particles decreases and may cause sudden and nonsymmetric settlements. These soils are found in warm, dry and desert areas. Iran is among the countries that have Collapsible soils. The necessity of investigation of these soils according to the relatively high development of these soils in different areas and requiring to design dams, irrigation canals and other technical buildings in these areas becomes much more important. Several studies have been conducted on the improvement collapsible soils but in this study we checked the collapse index of these soils by using sewage sludge ash and sewage sludge ash/lime. At first for testing these soils, undisturbed samples have been taken from Kalale of Golestan. Then remolded samples were made. The remolded samples had the same void ratio as undisturbed samples. In order to assess the behavior of these soils, saturated tests were done by standard consolidation device. The present study investigated the effect of the addition of sewage sludge ash and sewage sludge ash/lime on the collapse index of soil at field void ratio and different moisture content. Four samples that were tested are the addition of 0%, 2%, 4% and 8% sewage sludge ash and sewage sludge ash/lime to collapsible soil. The results showed that increasing the sewage sludge ash and sewage sludge ash/lime content of the soil decreases collapse index of soil. It was found that increasing the percentage of these materials will decrease the collapse index and also in disturbing soils, collapse index decreases even though the samples have been remolded in the same void ratio of undisturbed sample.