First principles calculations predict the promising thermoelectric material ZnGeSb2 with a huge power factor (S2σ/τ) on the order of 3 × 1017 W m-1 K-2 s-1, due to the ultra-high electrical conductivity scaled by a relaxation time of around 8.5 × 1025 Ω-1 m-1 s-1, observed in its massive Dirac state. The observed electrical conductivity is higher than the well-established Dirac materials, and is almost carrier concentration independent with similar behaviour of both n and p type carriers, which may certainly attract device applications. The low range of thermal conductivity is also evident from the phonon dispersion. Our present study further reports the gradual phase change of ZnGeSb2 from a normal semiconducting state, through massive Dirac states, to a topological semi-metal. The maximum power factor is observed in the massive Dirac states compared to the other two states. © 2016 the Owner Societies.