Phosphorene is a new-emerging two-dimensional material with many fascinating electronic and thermal properties. Using nonequilibrium Green's function technique, we investigate the thermoelectric transport properties of phosphorene in the ballistic transport regime. We find that while the electronic conductance and thermal conductance of phosphorene are highly anisotropic, the Seebeck coefficient is isotropic. The maximum predicted thermopower reaches 2500 μV/K. We also find that the Wiedemann–Franz law is valid only when the chemical potential is inside valence band or conduction band. When the chemical potential is near the valence band maximum or conduction band minimum; however, the Wiedemann–Franz law becomes invalid, and interestingly, the figure of merit ZT reaches its maximum value. We also find that figure of merit ZT increases with the increase of temperature, and ZT in the armchair direction is much higher than that in the zigzag direction. By analyzing the various effects on ZT, we discuss the possible routines to enhance figure of merit ZT.