Fly ash is widely accepted as a construction fill material. Utilization of fly ash as a backfill material in mechanically stabilized earth (MSE) walls is explored in the present study. The common modes of failure in MSE walls include (1) pullout of reinforcement, and (2) sliding of backfill material along the surface of the reinforcement. Interfacial shear tests are typically performed in a modified shear box to study the sliding mode of failure. In this study, ash from two different thermal power plants located in India-Neyveli Thermal Power Plant and Ramagundam Thermal Power Plant-Are used to study the interfacial shear properties between the fly ash material and the geogrid type of reinforcement. The interface properties of fly ash-geogrid are compared with that of conventional fill material-geogrid. The post peak loss of strengths is found to be less in the case of geogrid reinforced fly ash indicating the change in behavior of the fill material from brittle to ductile. The interfacial shear parameters, adhesion and interface friction angle, are found to range from 13.6-To-25.6 kPa and 33.8-To-45.5°, respectively. The interaction coefficient of fly ash and geogrid ranges between 0.87 and 0.92. The interface properties provided in the study may be adopted in the design of geogrid reinforced fly ash structures. © 2018 American Society of Civil Engineers (ASCE).All Rights Reserved.