Further utilization of waste heat produced by power plant system will increase the efficiency of the whole power plant system. The Kalina Cycle offers alternative solution to generate additional power from waste heat or from low temperature geothermal resources. The modeling application on energy system is used to study the basic design of thermal system, which uses Kalina Cycle for further utilization of geothermal brine water. If this geothermal brine water were not used in the Kalina Cycle, it would be reinjected to the geothermal reservoir and its remaining energy content would have been wasted. The study of this process is done by Cycle Tempo 5.0 simulation software, to obtain the data of efficiency, energy and exergy that could be generated from the heat source. An ammonia-water mixture is used as a working fluid on Kalina cycle system (KCS) 34. In order to obtain the maximum power output and maximum efficiency, the system will be optimized on the mass fraction of working fluid and also on the turbine output pressure. The result of the study shows that the maximum efficiency and power output are achieved at 78% ammonia-water mixture.