The performance of a distillator in a double – cycle distillation heat pump system, utilizing water as a working fluid in finned tube heat exchangers for evaporator and condenser, has been investigated. A mathematical model was set for the whole system and a simulating program was developed to simulate the double – cycle processes during the steady – state operation. This subroutine program to calculate the water and steam properties in the range of (65 – 115)oC and (0.25 – 1.6906)bar.The experimental results showed good agreement with the results obtained from the computer program. The concentration on the distillator performance during steady – state operation of the system is occurred, and it was found that the distillator performance (COPdist.) increased when the working mass flow rate increased and also it was increased when the working fluid pressure increased and had a maximum value (83.3%) at working mass flow rate of 5.5 kg/hr, working fluid pressure of 0.8 bar and process fluid pressure of 1.01825 bar.