A plate fin heat exchanger was optimally designed by selecting effectiveness and total annual cost as two simultaneous fitness functions using particle swarm optimization algorithm. Due to the variation of temperature and pressure in the exchanger passages, the non-uniform properties are occurred throughout the heat exchanger. To consider the effect of mentioned properties variation, the heat exchanger was segmented into sub-exchangers (for example 100) and properties were determined at the mean temperature for each small exchanger. The optimum results in the case of segmented (SEG) heat exchanger were compared with conventional properties evaluation or un-segmented (UN-SEG) heat exchanger in which the fluid properties are just determined at the mean temperature of outlet and inlet of heat exchanger. To generalize the results, the optimization was performed for the different hot stream inlet temperatures. The optimum results indicate that, the effectiveness decreases while the total annual cost increases in the case of SEG compared with UN-SEG. For example, total annual cost increases 2.8, 6.1, 11.7, 15.9 and 19.9%, respectively for the hot side inlet temperature of 400, 500, 600, 700 and 800 K, and for the heat exchanger with effectiveness of 0.8. Furthermore, effectiveness decreases about 1, 2, 3, 4.5 and 5.5%, respectively for the hot side inlet temperature of 400, 500, 600, 700 and 800 K. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.