This report presents the results of an impact assessment of rehabilitation intervention on irrigation system performance in the Gal Oya Left Bank irrigation system in Sri Lanka. Results indicate that rehabilitation had substantial overall impacts on increasing the irrigated area and decreasing the irrigation supply per unit area. Though the impacts of rehabilitation on productivity are indirect, a large increase in productivity is also seen after rehabilitation. It is also evident that the improvements in performance of these indicators in the Left Bank could not have resulted from physical or institutional improvements alone. Both physical and institutional components of rehabilitation intervention implemented simultaneously have contributed significantly to increasing the irrigated area. As reflected by this assessment, institutional improvements contribute significantly in reducing the excess irrigation supply to the Left Bank.