Spunbond fibrillation, an industrial scaled technology, is utilized in a wide range of applications such as medical products, textiles, and others. Here, an in-situ fibrillated method was proposed to fabricate the fiber-in-fiber polymer composite using spunbond technology. In the study, we investigated four researches: Chapter 2 explored the influence of morphology of PBT domains on the crystallization, rheological, and mechanical properties of PBT/HDPE composites; Chapter 3 investigated the effectiveness of the coupling agent on the surface compatibility between PBT and PE interfaces and its mechanical behaviour; Chapter 4 discussed the impacts of the UV curing time and the second phase content on the morphology, and mechanical behaviours of the EPDM/PE composites; and Chapter 5 studied the morphology of the E-beam cured EPDM/PE (and mPE/PE) composites. The in-situ fibrillar strengthened/ toughened composites are promising for applications such as automotive, astronomy, medical services, and consumer goods, and so on.