Some important characteristics of arid and semiarid soils are weak structure with bad physico-chemical properties. In these regions soil structure is affected by high exchangeable sodium percentage (ESP>15%). This condition increases soil dispersion and swelling potential and increases the rate of soil erosion. The role of amendment materials such as gypsum (CaSO4, 2H2O) in agriculture lands with high ESP has been clear. We evaluated the effect of quality and quantity of gypsum application on soil erosion and runoff in south Tehran's unstable sodic soils (marl soils). In this project for experimental design we used was a completely randomized blocks model analyzed with SPSS and MSTATC computer programs. The equivalent of 5 ton ha-1 of the following gypsum sources was added to soil: 1. Powder laboratory gypsum, 2. Powder industrial gypsum, 3. Suspension of gypsum, 4. Without any gypsum (blank). Each treatment has three slopes (10, 20 and 30%) and three replications (3×3×4=36 plots) and plot dimensions were 2 square meters (1m×2m). During this project 8 samples were collected after 8 rainfall events, then the amount of runoff, sediment and other soil physical and chemical properties were measured. Results indicated that: 1. Application of gypsum has significantly reduced runoff (40%) and sediment yield (35%). 2. Thjere was no significant defference between gypsum applications. 3. Application of gypsum had a significant effect on soil EC, but did not significant effect on soil acidity (pH). Key words: marl soils, gypsum, unstable soil, runoff, and sediment yield.