In 2016, bulk tellurium was experimentally observed as a remarkable thermoelectric material. Recently, two-dimensional (2D) tellurium, called tellurene, has been synthesized and has exhibited unexpected electronic properties compared with the 2D MoS$_2$. They have also been fabricated into air-stable and high efficient field-effect transistors. There are two stable 2D tellurene phases. One ($\beta$-Te) has been confirmed with an ultralow lattice thermal conductivity ($\kappa_L$). However, the study of the transport properties of the other more stable phase, $\alpha$-Te, is still lacking. Here, we report the thermoelectric performance and phonon properties of $\alpha$-Te using Boltzmann transport theory and first principle calculations. A maximum ZT value of 0.83 is achieved under reasonable hole concentration, suggesting that the monolayer $\alpha$-Te is a potential competitor in the thermoelectric field.