This paper reports a numerical study on the extinction limits and flame structures of opposed-jet syngas diffusion flames. A narrowband radiation model is coupled to the OPPDIF program, which uses detailed chemical kinetics and thermal and transport properties to enable the study of 1-D counterflow syngas diffusion flames over the entire range of flammable strain rates with flame radiation. The effects of syngas composition, strain rate, ambient pressure, and dilution gases on the flame structures and extinction limits of H2/CO synthetic mixture flames were examined. Results indicate the flame structures and flame extinction are impacted by the composition of syngas mixture significantly. From hydrogen-lean syngas to hydrogen-rich syngas fuels, flame temperature increases with increasing hydrogen content and ambient pressure, but the flame thickness is decreased with ambient pressure and strain rates. Besides, the dilution effects from CO2, N2, and H2O, which may be present in the syngas mixtures, were studied. The flame is thinner and flame temperature is lower when CO2 is the diluents instead of N2. The combustible range of strain rates is extended with increasing hydrogen percentage and ambient pressure, but it is decreased the most with CO2 as the dilution gas due to the dilution effects. Complete flammability limits using strain rates, maximum flame temperature as coordinates can provide a fundamental understanding of syngas combustion and applications.