Abstract The stability of a blown film extrusion process has been investigated using a simple isothermal Newtonian model. Although the Newtonian model may be an oversimplification for polymer melts, it serves as a reasonable first approximation for linear polymers (e.g., linear low density polyethylene) in extensional flow situations. The results obtained by the method of linear stability analysis indicate that the blown film process becomes unstable if the blow-up ratio (BUR) or the film thickness reduction is larger than a certain critical value. In addition, existence of an optimum freeze-line-height is predicted at which the region of stable operation is largest in terms of the BUR and the film thickness reduction. These predicted trends are in qualitative agreement with experimental observations for linear polymers such as LLDPE.