Despite the popularity of fiber reinforced polypropylene (PP) composites both in academia and industry, the research on basalt fiber (BF) reinforced thermoplastic PP composites is quite limited. Due to their superior properties, BFs have a great commercial potential for industrial applications. In this study, six different silane coupled BF/PP composites were prepared with fiber contents up to 15wt% using a twin-screw extruder. Maleic anhydride grafted PP (MA-g-PP) coupling agent content was fixed at 10wt%. Mechanical properties were investigated by tensile, flexural, and Izod impact tests. Thermal, viscoelastic, and morphological properties were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM), respectively. The results showed that BFs have substantial effect on the mechanical and viscoelastic properties of the composites. The considerable increase in the tensile strength, flexural strength, and Izod impact strength was attributed to the higher interfacial adhesion between the BF and PP matrix in the presence of MA-g-PP coupling agent. The SEM images proved the increased interfacial adhesion between the fibers and the PP matrix. The improved interfacial adhesion was also affected the crystallization behavior of PP.