We present the preparation of polyamide 6 (PA6)/Vapor Grown Carbon Fiber (VGCF) composites by melt compounding. Two commercialized forms of PA6 with different melt flow rates (herein named PA6-low and PA6-high.) and two VGCF, VGCF-H (aspect ratio: 40) and VGCF-S (aspect ratio: 100) were used in this study. Young's modulus and the yield strength increased when small amount of VGCF (e. g. 1 wt. %) was added to PA6, and the properties of PA6-low/VGCF composites increased with a decrease in the aspect ratio of VGCF. According to scanning electron microscopy (SEM) observations, the dispersion state of VGCF in PA6-low/VGCF-S composites is superior to the PA6-low/VGCF-H composites. This may be due to the weakening of tensile properties caused by cracks between the adjoining crystals generated by the large amount of VGCF that act as nucleon's. Hence, the aspect ratio of VGCF has an influence on the tensile properties of PA6/VGCF composites. The mechanical properties were shown to increase with VGCF-H length. Matrix viscosity and the screw rotation speed were also found to influence the mechanical properties of PA6NGCF-H composites.