There are two degrees of freedom available in designing an Organic Rankine Cycle (O.R.C) - the organic fluidand the evaporator pressure. The upper limit of the latter will be decided by the heat source inlet temperature.The performance of 12 organic fluids including some newly developed silicone oils - D4, MDM, have beenanalyzed on the basis of thermal efficiency, exergy destruction (neglecting pressure drop) and size of the systemrequired. The optimum evaporator pressure was found for each fluid using power output as the objectivefunction. The source was taken as a flue gas mixture at 150˚C. The exergy destruction due to two irreversibility–heat transfer across a finite temperature difference and pressure drop in a shell and tube heat exchanger wasquantified only for R245fa and recalculated for varying source exit temperature. The average tube side heattransfer coefficient and that for liquid-only were obtained from HTRI software for a given geometry. Thepressure drop in the shell was obtained using Bell-Delaware method and that in the tube was calculatedseparately for the single phase and two phase part.