Lowering the dimension of 3D materials, so that the confinement length L of the low-dimensional material is less than the thermal wavelength Λ of its bulk phase, is expected to be a sufficient way to enhance their thermoelectric performances. Using density functional theory incorporating the linearised Boltzmann transport equation with a constant relaxation time approximation, we calculate the electronic and thermoelectric properties of monolayer and bilayer XTe (X = Ge, Sn, and Pb). It is shown that the ideal figure of merit of monolayer XTe is larger than that of bilayer XTe, suggesting the importance of downsizing the bulk XTe up to single-layer thickness to have a better thermoelectric performance. The n-type monolayer buckled SnTe is predicted to exhibit remarkable thermoelectric performance with ZT > 1.6 at T = 900 K compared to other monolayer and bilayer XTe.