This research evaluates the use of natural fiber in polyamide-6 composites aiming to synthetic fiber replacement. The application of natural fiber as reinforcement in composite materials requires a strong adhesion between fiber and the synthetic matrix. In this work, the surface treatments were mercerization and applying the silane coupling agent. The Vetiver grass fiber reinforced polyamide-6 composites at various contents of Vetiver grass fiber (15 and 30%) were prepared by using twin screw extruders. The mechanical properties, morphology and long term stability of Vetiver grass fiber reinforced polyamide-6 were investigated and compared with talc filled and glass fiber reinforced composites. Polyamide-6 composites reinforced with grass fiber treated with silane coupling agent had improved properties compared with grass fiber treated by mercerization. The comparison of tensile properties of the treated grass fiber reinforced polyamide-6 composites with unfilled composite showed that the mercerization and silane coupling agent of treated grass fiber gave a significant increase in tensile strength. The results also showed that the mercerization and silane coupling agent could significantly improved the flexural strength and modulus of grass fiber reinforced composites. After accelerated weathering, grass fiber reinforced polyamide-6 composites retained a higher fraction of the original mechanical properties than unfilled, talc filled and glass fiber reinforced composites.