Thermoelectric power generation is one of important technologies for realization of highly efficient energy uses. Improvement of performance of thermoelectric materials is required, and theoretical studies suggest one-dimensional materials have potentials for the realization of the highest performance [1]. However, how 1D materials can really improve the thermoelectric performance has not been experimentally verified yet. In this context, thermoelectric properties of single walled carbon nanotubes attract a lot of attention because of their one-dimensional structure and electronic structures (See a review by Blackburn et al. [2]). Thermoelectric properties significantly depend on the electronic structures of SWCNTs, and relationships among Seebeck coefficient (S), electrical conductivity (σ), power factor, doping levels and their electronic structures have been revealed through various studies [3-5]. As a result, a very unique characteristics on thermoelectric properties of SWCNTs has been clarified [5]. In conventional materials, there is a trade-off between Seebeck coefficient and electrical conductivity, but in the metallic SWCNTs, the trade-off is broken because of the presence of 1D electronic structures, indicating the uniqueness of SWCNTs [5]. For the correct understanding of the thermoelectric performance, zT, in SWCNT thin films, clarification of the relationships among S, σ, and thermal conductivity κ is very important. Although the relationships between S and σ have been well understood, the relationships among those parameters and κ have not yet. Avery et al. clarified κ is almost constant by the change of σ, and they estimated the zT value is approximately 0.01-0,05 [3]. Recently we developed a technique to clarify the cross-plane thermal conductivity κ ⊥ in SWCNT thin films with tuned Fermi level by a combination of time-domain thermoreflectance method and electrolyte gating technique [6], and then clarified that the κ ⊥ is almost constant even when in-plane σ is changed three order of magnitude from ~10 Sm-1 to 104 Sm-1 [5]. Such constant behavior of κ by the change of σ in SWCNT thin films is interesting in both basic science and application perspectives. Now we can prepare Fermi-level tuned and microscopically aligned SWCNT thin films [7], and the relationships between S and σ in the aligned directions have been clarified [8]. In this talk, I would like to discuss the recent results in our group and the estimated zT values of Fermi-level tuned and aligned SWCNTs.