Expansive soils have the inherent property of swelling when they are adsorbed water and shrinking when they are dried. This property of expansive soil causes to damage structures constructed on it. To minimize the impact of the expansive soil, it is necessary to improve existing soil before commencing infrastructural construction activity. This study aims palm tree leaf ash as a partial replacement of lime for the improvement effect of expansive soil on the soil water characteristics curve. From three test pits, six soil samples were collected from Bahir Dar City and tested for identification and characterization. The soil was improved with lime content of 3%, 5%, 7%, and 9% by the dry weight of the soil. The results show that the optimum lime content was 7% and half of the optimum lime content was replaced by palm tree leaf ash with the amount of 5%, 10%, 15%, and 20%. The optimum combined additive decreases the plasticity index from 66.04% to 19.59%, free swell index from 130% to 37.5%, maximum dry density from 1.36 g/cm 3 to 1.29 g/cm 3 , California bearing ratio - swell from 11.12% to 1.72%, and increases the unconfined compressive strength from 184.29 kPa to 523.10 kPa, California bearing ratio value from 1.90% to 21.10% at 3.5% lime and 15% palm tree leaf ash. Soil water characteristics curve renders information on the amount of water in the soil and an adequate for analyzing unsaturated soil mechanics problems containing soils that keep on under partially saturated conditions. The SWCC was determined by using pressure plate apparatus with the suction value of 33, 200, 400, 800, 1000, 1200, and 1400 kPa. The results of measured laboratory discrete data points were fitted to SWCCs by using the Fredlund and Xing (1994) model equations. This study reveals that the SWCC fitting parameters ‘‘a’’ and “m’’ were decreased while parameter “n’’ was increased when the expansive soils are improved due to the accumulation of clay particles caused by cation exchange between stabilizers and clay soil particles by forming of more open, hard, and aggregated soil structure. Keywords: Expansive Soil, Palm tree leaf ash, Soil improvement, Soil Water Characteristic Curve.