This paper presents the results of an experimental study of the unstable capillary flow of HDPE/PA 11 blends with special emphasis on the pressure oscillation legion. The main result is the finding that addition of only 1% by weight of the PA 11 component to HDPE completely, suppresses the oscillation phenomenon normally found with the latter polymer when the measurement is carried out in a rheometer with a plane capillary entrance. In comparative experiments with a capillary rheometer with a conical (120 degrees) entrance, the pressure oscillation effect disappeared at a PA 11 content of＇ 5%. In compounds containing 1, 3 and 7 % PA 11, distinct pressure oscillations were recorded in the latter case. The stress and elongation at break of the extrudates change in a monotonic fashion with the composition. SEM photographs and DSC measurements show that the two components are basically immiscible.