Abstract The in-plane phonon-drag thermopower S ^g, diffusion thermopower S ^d and the power factor PF are theoretically investigated in a twisted bilayer graphene (tBLG) as a function of twist angle θ, temperature T and electron density n _s in the region of low T (1–20 K). As θ approaches magic angle θ _m, the S ^g and S ^d are found to be strongly enhanced, which is manifestation of great suppression of effective Fermi velocity v _F ^* of electrons in moiré flat band near θ _m. This enhancement decreases with increasing θ and T. In the Bloch–Grüneisen (BG) regime, it is found that S ^g ∼ v _F ^*−2, T ³ and n _s ^−1/2. As T increases, the exponent δ in S ^g ∼ T ^δ , changes from 3 to nearly zero and a maximum S ^g value of ∼10 mV K⁻¹ at ∼20 K is estimated. S ^g is found to be larger (smaller) for smaller n _s in low (high) temperature region. On the other hand, S ^d, taken to be governed by Mott formula with S ^d ∼ v _F ^*−1, T and n _s ^−1/2 and S ^d ≪ S ^g for T > ∼2 K. The power factor PF is also shown to be strongly θ dependent and is very much enhanced. Consequently, possibility of a giant figure of merit is discussed. In tBLG, θ acts as a strong tuning parameter of both S ^g and S ^d and PF in addition to T and n _s. Our results are qualitatively compared with the measured out-of-plane thermopower in tBLG.