The reinforced and toughened PP/PA66 in situ composites were prepared via extrusion-drawing-injection method. The relationship among composition, phase morphology and mechanical properties, together with their functional mechanism, was investigated. The results show that in the range of PA66 weight fraction (f(w)) from 0 to 20% and under the experimental processing conditions, the main changes in phase morphology of the composites with fw are that the number of in situ formed PA66 microfibers and remained PA66 particles increases with f(w) whereas the fiber transverse size and its dispersity decrease till f(w) = 15% and then increase. This can be attributed to the combined effect of break-up, coalescence and deformation of the PA66 phase in the PP phase in the course of extrusion and drawing. The tensile strength of composites has a maximum value at f(w) of 15% and Young's modulus increases with f(w) up to a plateau level, while impact strength continuously rises with f(w), an effect which can be ascribed to the distinct dependence of these properties on the phase-morphological factors mentioned above. (C) 2003 Kluwer Academic Publishers.