Pavements and embankments fail for different reasons; poor design, poor materials and poor construction methods are the most common. The pavement foundation (subgrade) represents one of the key elements in pavement design and its behavior shall influence the overall pavement performance. It was found necessary improving gypseous subgrade stiffness characteristics in order to prolong pavement design life. We also believe that too much emphasis in finding out untraditional methods in treating weak gypseous soils and then using them in subgrade layers taking into consideration type and values of loading effects in the employed design models. This has lead to erroneous predictions. In general, the more resistant to deformation the subgrade is the more loads it can support before reaching a critical deformation value. Gypseous soils cover about (35%) of the total area of Iraq and therefore the study of the properties of these soils to be used as subgrade embankments becomes very important due to the problems caused by these soils during leaching which lead to what is called collapsibility. Many trials were conducted in order to study the possibility of improving the properties of gypseous soils using chemical and natural materials. Results showed that some natural and chemical materials could improve the behavior of gypseous soils, but these results are based on routine laboratory tests. In this study, two types of local gypseous soils are selected to be improved by three chemical stabilizing materials, namely, 5% hydrated lime, and 2.5% hydrated calcium chloride and 6% kaolin. The soils employed are classified as (SP-SM) and (SP) with gypsum content of (48.3%) and (35%) for Baiji and Al-Thurthar area soils, respectively. An engineering model is adopted to carry out tests on these soils and to analyze their behavior before and after improvement especially their collapsibility property. The results show that all employed chemical and natural additives can be used to reduce the collapsibility property of gypseous soils, but with different degrees. Using lime reduces the collapsibility of gypseous soils to a small degree compared with calcium chloride and kaolin.