As the practical demand for flexible power-conversion devices increases, the development of high-performance alternatives to brittle inorganic thermoelectric (TE) materials is essential. Organic polymers, nano-carbons, and their hybrid materials have been investigated as possible alternatives for flexible TE materials due to their well-known advantages including flexibility, light-weight, low cost, easy processability, and scalability. Among them, carbon nanotubes have great potential for use in flexible TE materials because of their high electrical conductivities and controllable Seebeck coefficients. Here, we show the rationally designed flexible carbon nanotube yarns with excellent TE performance. The carbon nanotube yarn exhibits a superior electrical conductivity due to increased longitudinal carrier mobility derived from a highly aligned structure. Furthermore, our TE generator is innovative in that the carbon nanotube yarn acts as multifunctions in the same device.