Gypseous soil is one of the soils that suffer from problems and suffers from a reduction of shear strength and collapse when exposed to water immersion or water filtration in it. Many researchers have tried to solve these problems in different ways and by using many materials as additives to improve the performance and efficiency of this soil. In this research, the behavior of soil with a high content of gypsum (61.49%) is examined, using iron slag which is a by-product of the iron making process in melting furnaces, used as an additive in proportions (2, 4, 6, 8, 10., 12) %, by dry mixing method with soil. Tests are carried out to determine the effect of this substance on the shear strength parameters. The effect of water immersion on soil cohesion (c) is reduced until it reaches (c) in the case of immersion a value very close to the value in the dry state at slag ratio (10) %. This is the optimum ratio of slag to improve the value of (c). Whereas for the value of (ø), with the increase of the slag rate for both wet and dry cases, the value of (ø) increases, where (ø) reaches the highest value at; the slag rate (10) % for the dry state, and the slag rate (8) % for the soaked case. Whereas for the collapse potential (Cp), adding the slag reduces the value of the soil collapse potential (Cp), from (10.6) to the soil without additives until (0.95) for the slag rate (12) %. Then the soil becomes problematic soil.