This study examines possibility of improving clayey soils in the Handere Formation exposed in the vicinity of Adana (S. Turkey), one of the largest cities in southern Turkey. The Handere Formation, from where the samples for this study are taken, is stratigraphically at the upper most part of the marine sediments of the Adana Basin. The unit is located at the northern part of Adana city. The samples were examined in the geotechnical laboratory to determine the effect of plaster mortar (Master Cast) on the geotechnical properties of the soil and its ability to improve the soil. shear box, consolidation, unconfined compressive strength tests were applied on the samples. It has been shown that the master cast used can improve soil properties geometrically and can be used as a soil stabilizer. The plasticity values of the soils were reduced by master cast addition. Besides, it was determined that the highest maximum dry unit weight and the lowest optimum moisture content were obtained by 15% master cast addition. The soil strength properties were reached to the top values in case of 15% master cast added into the mixtures. Besides, it is determined that the coefficient of volume compressibility (Mv) and the pre-consolidation pressure values are ideal when the master cast ratio in the mixtures are 10% and 5% respectively.