This paper systematically investigates the effect of a number of geometric parameters on the texturing performance of air-jet texturing nozzles. In order to facilitate the research, an air-jet texturing nozzle with a rectangular cross section has been developed. The texturing performance of the nozzles is assessed by means of process observations and on-line measurement of stabilizing zone tension, and also by measuring the increased linear density of the yarns on textured yarn samples. Furthermore, instability, elongation at break, and tenacity are measured, and texturing quality is judged by visual inspections and examination of scanning electron microscopy images of the textured yarns. Tension in the stabilizing zone, increase in linear density, and to a somewhat lesser extent instability are reliable measures of texturing quality. The best texturing comes from nozzles with a slightly diverging main channel and a single air inlet hole located far from the nozzle exit. A curved diverging exit profile is essential for successful texturing. The results of the tests to determine the effect of air inlet angle are inconclusive and require further investigation.