To understand the behavior of thermoelectric efficiency of single-molecule junctions from 0 K to room temperature, we investigated the thermoelectric properties of a dibenzenedithiol (DBDT) single-molecule junction. We investigated its Seebeck coefficient (S), electric conductance (σ), and electron's thermal conductance (κel) in the framework of parameter-free density functional theory combined with the Lippmann-Schwinger formalism in scattering approach. We observe that the nanojunction is p-type and the value of the Seebeck coefficient at room temperature is around 40 μV/A, in agreement with the results of the experiment. In addition, we investigate the phonon's thermal conductance (κph) using (i) the weak-link model suitable for ballistic phonon transport mechanism in the low-temperature quantum regime and (ii) the nonequilibrium molecular dynamics (NEMD) simulation in the high-temperature classical regime. We finally construct the phase diagram for ZT, where the value of ZT reveals the power law behavior that falls into four phases because of the competition between κel and κph and the crossover from the quantum to classical phonon transport mechanism for κph. Our theory shows the following ZT ∝ Tx where x = 2, 0, 2.26, and 3 in different temperature regimes labeled by I, II, III, and IV, respectively.