In this study, we introduced a single-stage in-situ fibrillation of 2, 5, and 10 wt% of polyamide (PA), polybutylene terephthalate (PBT) and polyethylene terephthalate (PET) within a high molecular weight semi-crystalline polylactic acid (PLA). This was via reducing the twin screw extruder (TSE) processing temperature to very low temperatures below the melting temperature of the second phase and above that of PLA. A high molecular weight amorphous PLA was also used as the matrix in the PLA-PA system. In all systems, the processing was carried out at 200oC and 600rpm. The testing specimens were produced by compression molding and the mechanical, thermal and viscoelastic properties of the samples were investigated using tensile test, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and rheometer. According to the results, in samples that the in-situ fibrillation could be induced improved properties of PLA was observed.