Thermal conductivities of bismuth telluride thin films were measured by using 3ω method. The measured thermal conductivities depend on the grain size of the film controlled by post annealing. Spectral phonon model is considered to evaluate the thermal conductivities of nanocrystalline bismuth telluride thin films. The decreasing thermal conductivity with decreasing grain size was roughly explained although the spectral model includes some assumptions for simplicity. The phonon mean free path calculated by the spectral phonon model is relatively long, and much longer than the mean free path of electron. Bismuth telluride porous thin films were fabricated to cut the ballistic phonon transport with long mean free path as well as keeping diffusive electron with short mean free path. The measured thermal conductivity of porous thin film was extremely low, and the non-dimensional figure of merit was enhanced by sub-micron structures.