The present work concentrates on the effect of epoxy surface treatment and twisting on mechanical and interfacial properties of unidirectional (UD) fiber reinforced thermoplastic composites. The purpose of study is to find out whether the mechanical and interfacial properties of composites can be improved by surface treatment and twisting or not. In all specimens, PA 66 was commingled with aramid (AR) yarn. AR was used as reinforcement and PA 66 was used as resin yarn. The different weight percent of epoxy, such as 3, 6, and 9wt% and twisting such as 0, 20, 40, 60, 80, and 100tpm, were applied on the commingled yarns. After fabrication of UD composites with commingled yarns, the mechanical properties of laminates were investigated by conducting tensile and three-point bending test. Consequently, the optimum twisting number and wt% of epoxy surface treatment was found. By increasing twisting until an optimum level, the tensile strength of fiber bundle as well as tensile and bending strength of UD composite increased. The interfacial properties of composites with various twisting levels did not change.