This paper concerns the morphology development of in situ compatibilized semicrystalline polymer blends in a co-rotating, intermeshing twin-screw extruder, using polypropylene (PP) and polyamide 6 (PA-6) blends as model systems. The morphology of in situ compatibilized blends develops much faster that of mechanical ones. The size of the dispersed phase (PA-6) undergoes a 10(4) fold reduction from a few millimeters to sub-micron during its phase transition from solid pellets to a viscoelastic fluid. The final morphology is reached as soon as. the phase transition is completed, which usually requires only a small fraction of the screw length in a co-rotating twin screw extruder. Screw profiles and processing conditions (screw speed, throughput and barrel temperature) control the PA-6＇s melting location and/or rate, but do not have significant impact on the ultimate morphology and mechanical properties of in situ compatibilized blends. The finding that morphology of PP/PA-6 reactive blend develops rapidly makes it possible to produce compatibilized PP/PA-6 blends by the so-called one-step reactive extrusion. It integrates the traditionally separated free radical grafting of maleic anhydride onto PP and the compatibilization of PP/PA-6 into a single extrusion step.