This study investigates the geothermal reservoir of Balçova geothermal field using the program Fluent which is written for general problems of fluid flow and heat transfer in a given complex geometry. Geothermal reservoir simulation of Balçova geothermal field is made by Istanbul Technical University, Department of Petroleum and Natural Gas Engineering using the program Though2 which is written for geothermal reservoir application in 2001. The results of this study become a starting point of this thesis. Besides, the new techniques applied the geothermal field since that period, and reinjection of the geothermal fluid by the well BD-8 to the reservoir, made it necessary to remodel of the field. During the modeling study, the conceptual model of the field is developed based on technical data and advices by the Balçova Geothermal Ltd. The fault Agamemnon-I existing in the field is thought the dominated fault of the reservoir and assumed that the heated water from the aquifer is raised to the surface using that fault as a flow path. The geometry of the conceptual model of the reservoir is drawn and meshed properly using Gambit meshing program and meshed reservoir is exported and run in Fluent program under given boundary conditions and after that 3-D temperature distribution of the reservoir is obtained.