The effect of spinning process conditions on the permeation and mechanical properties of asymmetric hollow fiber membranes based on a DuPont family of aromatic polyamides is discussed with particular emphasis on the role of gap-spinning dynamics and spinline stress profile. To gain insight of the spinline dynamics, the velocity profile of a spinning polyaramide fiber was measured as a function of distance in the air gap by using non-contact laser-Doppler velocimetry. A theoretical analysis of the measured velocity profile indicates that the spinning extensional viscosity of thr polyaramide spin dope increases as the rate of extensional strain in die air gap increases. The significance of this observation for the development of membrane properties is discussed. Increase of the air gap draw ratio or rate of extensional strain was found to render the morphology of the hollow fiber membranes denser and enhance both their He/N-2 selectivity and mechanical properties significantly.