The possibility of exploiting low-temperature heat sources has been of great significance with ever increasing energy demand. Optimum and cost-effective design of the power cycles provide a means of utilization of low-temperature heat sources which might otherwise be discarded. In this analysis, the performance of the Kalina cycle system 11 (KCS11) is examined,for low-temperature geothermal heat sources and is compared with an organic Rankine cycle. The effect of the ammonia fraction and turbine inlet pressure on the cycle performance is investigated in detail. Results show; that for a given turbine inlet pressure, an optimum ammonia fraction can be found that yields the maximum cycle efficiency. Further the maximum cycle efficiency does not necessarily yield the optimum operating conditions for the system. In addition, it is important to consider the utilization of the various circulating media (i.e., working fluid, cooling water and heat resource) and heat exchanger area per unit power produced. For given conditions, an optimum range of operating pressure and ammonia fraction can be identified that result in optimum. cycle performance. In general, the KCS11 has better overall performance at moderate pressures than that of the organic Rankine cycle.