Carbon nanotube (CNT) yarn is enables CNTs to be used on macro scale. However, the mechanical properties of CNT yarns are smaller than CNT itself, and improvement of the mechanical properties is a challenge for practical application. In this study, untwisted CNT yarns were fabricated by a dry spinning method, and the yarns were graphitized and combined with polymer for the purpose of development of CNT yarns with high strength. As a result of the graphitized treatment to the as-received yarns under inert atmosphere at 2800°C, impure materials and defect structure on CNTs were removed and strength of the yarn was increased by 19%. After combining the as-received yarns with polyacrylic acid (PAA), the strength was increased by 174% and reached 2.3 GPa. Breaking form of the yarns were changed from pulling out of CNT bundles to rapture of CNT bundles by graphitization and combining with PAA, indicating an increase interactive force between the CNT bundles. However, the strengthening effect was limited when graphitized CNT yarns were combined with PAA. As a result of molecular dynamics simulations, it was revealed force transfer capability of PAA was low when the graphitized yarns was combined with PAA. There were functional groups on as-received CNT such as carboxyl groups. On the other hand, the functional groups were removed from CNTs after the graphitization treatment. Consequently, interaction such as hydrogen bond between as-received CNT and PAA was removed by the graphitization, and it lead to the decrease of the force transfer capability of PAA.