The research is carried out for the optimal design of deethaniser process for natural gas separation. In order to calculate physical properties of low boiling components in supercritical state at normal operating conditions, Peng-Robinson equations are modified based on the design and operating data. The physical properties from the modified Peng-Robinson equation shows less than 3% deviation comparing with actual design and operation data. Steady state simulation is performed for conventional deethanizer process as well as optimal deethanizer process using PRO/II. From the view point of energy saving, pinch technology is utilized to evaluate capacity of cold box system. Using the simulation results, capital cost for each process is evaluated based on US Houston Gulf Cost and operating cost is calculated in the basis of utility consumed. As a result, the capital cost and operating cost for the proposed process is reduced 12.3% and 10.2% respectively.