The present paper deals with a theoretical study of condensing and cogeneration plants operate at partial load with constant and sliding (variable) live steam pressure controls. In this work, two Iraqi condensing units as well as two cogeneration with steam back-pressure turbine were chosen. These units are K-66-87-0.07, K-55-58.8-0.083, R-100-130-15 and R-40-130-29 respectively. A computer program had been written to work under MathCad 15 software to simulate these units under design and off design regimes with both types of control at nozzle and throttling steam distribution. The performance of the different schemes is analyzed in view of the first and second laws of thermodynamics. The results show that the selection of control type mainly depends on type of steam distribution. So, the heat rate (k) increases in condensing units with sliding live steam control and nozzle distribution according to first law of thermodynamics. The value of increasing (k) is about (0-6%) depending on the operation regime. While, this type of control with throttling steam distribution causes decreasing (k) in about (0-1%). Cogeneration units with back-pressure steam turbine operate only with nozzle distribution. So, the results show that using sliding live steam pressure control is associated with increasing heat rate (k), especially when ratio of flow rate is ≤ 0.9. This type of control for cogeneration units also causes increasing heat process directed to heat consumer and decreasing power to heat process ratio (α). According to the second law of thermodynamics the irreversibility losses were redistributed depending on control type.