Thermoelectric figure-of-merits (ZT) are enhanced or degraded by crystal defects such as twins and excess atoms that are correlated with thermal conductivity (k) and carrier concentration (n). For Bi2Te3, it is unclear whether the crystal defects can enhance ZT without a degradation in the thermopower factor. In the present study, n-type Bi2Te3 nanowires (NWs) are electrochemically synthesized to have twin-free (TF) or twin-containing (TC) microstructures with a ZT of 0.10 and 0.08, respectively, at 300 K. The ZTs of TF and TC NWs remarkably increase up to 0.21 and 0.31, when heat-treatments cause n-reduction and twins induce phonon scattering, as follows: first, the enhancement of the Seebeck coefficient from -70 to -98 mu V K-1 for TF NWs and from -57 to -143 mu V K-1 for TC NWs, by virtue of n-reduction; secondly, twin-driven k-reduction from 1.9 to 1.4 W m(-1) K-1 of TC NWs, while the k of TF NWs increases from 2.3 to 2.6 W m(-1) K-1 due to the enhanced carrier mobility. The lattice thermal conductivities of TC NW are lowered from 1.1 to 0.8 W m(-1) K-1 by phonon scattering at twins. Density functional theory calculations indicate that twins do not significantly influence the Seebeck coefficient of Bi2Te3. It is strongly recommended that twins be incorporated with an optimum carrier concentration to enhance the ZT of Bi2Te3.