Expansive soils change their volume due to changes in moisture condition. The volume change characteristics of this soil lead to different problems for pavements and other structures constructed on it. To minimize the problems related to these soils different mitigation measures such as stabilization of soils with different additives may apply. But the selected stabilization techniques should be economical in addition to being effective. Currently the costs of additives like cement and lime increases, it is best to try other locally available materials as a stabilizing agent. Therefore this research is focused on investigation of the effect of the addition of palm tree leaf ash on stabilization of expansive soils. The soils collected from Bahir Dar City are classified as A-7-5 according to AASHTO and based on USCS test pit-1 and test pit-2 soils are classified as CH and test pit-3 as MH. For evaluating the effectiveness of palm tree leaf ash /PTLA/ as a stabilizer test pit-1 materials that is highly expansive than the other test pits are used. The natural soil was stabilized with 2.5%, 5%, 10%, 15%, 20% and 25% of palm trees leaf ash by dry weight of the soil. Atterberg limits, linear shrinkage, free swell, specific gravity, compaction, unconfined compressive strength and California bearing ratio tests are conducted. The free swell index value reduced from a value of 120 % to 44 % with the addition of 25 % of palm tree leaf ash, it shows that the soil change from high swelling potential to low swelling potential at this amount. The plasiticty index and linear shrinkage value reduced from a value of 54.71% to 24.78% and from 19.72% to 9.61% respectively at 14 days curing period with the addition of 25% of palm tree leaf ash. But the plasticity index and linear shrinkage test result of the treated soil shows that the treated soil is still in the range of moderate degree of expansiveness at this amount. Similarly the maximum dry density decreases from a value of 1.56 g/cm3 to 1.27 g/cm3 and the optimum moisture content increases from 27.5% to 39.2% at 25%. The unconfined compressive strength increases from a value of 173.40kPa to 343.05kPa with the addition of 25% of PTLA. The soaked CBR value of the soil increases from 2.10% to 4.39% and the CBR-swell value decreases from 10.14% to 4.08% at 25% of PTLA. The laboratory test results shows that with the addition of the palm tree leaf ash upto 25% to the natural soil, the palm tree leaf ash by itself is not effective for treating expansive soils in the study area.