The air-flow field in melt blowing plays a key role in fiber drawing and nonwoven web performance. A multiobjective optimization using genetic algorithms was proposed to obtain optimum air-flow field with the lowest velocity decay and temperature decay of the air-flow field of a melt-blowing slot die. Four main geometry parameters, including slot width, head width, slot angle, and setback, were studied. The optimal results were achieved in the 50th generation with 20 individuals of each generation. The results also show that a smaller slot angle and larger slot width resulted in a lower air velocity decay and temperature decay. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 3520-3527, 2011