Compatible and partly-biobased poly(l-lactide) (PLA)/polyamide-12 (PA12) blends (30 wt% PA12) are here processed by twin-screw extrusion at high shear without added compatibilizers and the effect of several processing parameters (screw speed, feed rate) on final properties (tensile strength, ductility, impact toughness, thermal resistance) is addressed. High tensile strengths could be maintained for these blends with a maximal ductility (225%) and impact toughness (48 kJ/m2) achieved for an optimal screw speed of 800 rpm. However, extreme screw speeds higher than 800 rpm dramatically reduce ductility and impact toughness. Concerning thermal resistance, a constant increase of the heat deflection temperature is observed with the screw speed and thermal resistance up to 123°C could be obtained. In this respect, processing conditions of PLA/PA12 blends have profound positive effects on all (thermo)mechanical properties. Blend morphologies were revealed by scanning electron microscopy and refined PA12 fibrils are detected at optimal processing conditions. Properties deterioration were correlated to a PA12 fibrillar-to-ellipsoidal shape transition at extreme screw speeds arising from PLA degradation and attesting for the importance of the PA12 fibrillation process on final properties. Consequently, PLA/PA12 blends represent interesting biobased candidates for high-performance applications and their optimization could be easily performed by playing with extrusion conditions without compatibilizers. © 2020 Society of Plastics Engineers