Polypropylene/poly(butylene terephthalate) (PP/PBT) microfibrillar composites (MFCs) were prepared through in situ one-step direct extrusion using a triangle-arrayed triple-screw extruder (TTSE) at a processing temperature between the melting points of two components. The orthogonal experimental method was designed to assess the effects of the PBT content and processing parameters (temperature, screw speed, and feed rate) on the morphology of PBT fibrils. It can be found that the most important influence factor on the fibril formation is the PBT content, followed by temperature, feed rate, and screw speed. Furthermore, the morphological evolution procedures of the dispersed phase started from spherical pellets to ellipsoids or ribbons, forming short fibrils, and consequently high-aspect-ratio fibrils appeared under the alternating shear-extensional flow field. Moreover, the rheological properties of linear PP incorporating fibrillated PBT were thoroughly investigated. The relaxation time of blends with various fibrous PBT was linearly proportional to the aspect ratio of fibrils. Strain-hardening in extensional flow was observed for blends with long fibrils, and the strain-hardening factor grew with the fibrillar aspect ratio, indicating the formation of a physical entanglement network between fibrils and matrix. POLYM. ENG. SCI., 58:1166-1173, 2018. (c) 2017 Society of Plastics Engineers