Partial oxidation of methane to synthesis gas was carried out using supported iridium-nickel bimetallic catalysts, in order to reduce loading levels of iridium and nickel, and to avoid carbon deposition on nickel-based catalysts by adding iridium. The performance of supported iridium-nickel bimetallic catalysts in synthesis gas formation depended strongly upon the support materials. La2O3 gave the best performance among the support materials tested. Ir(0.25 wt%)-Ni(0.5 wt%)/La2O3 afforded 36% conversion of methane (CH4/O-2=5) to give CO and H-2 with the selectivities of above 90% at 800 degrees C, and those at 600 degrees C were 25.3% conversion of methane and CO and H-2 selectivities of about 80%, respectively. Reduced monometallic Ir(0.25 wt%)/La2O3 and Ni(0.5 wt%)/La2O3 catalysts did not produce synthesis gas at 600 degrees C. A higher conversion of methane was obtained by synergistic effects. The product concentrations of CO, H-2, and CO2, and CH4 conversion were maintained in high values, even increasing the space velocity of feed gas over Ir-Ni/La2O3 catalyst, indicating that rapid reaction takes place. As a by-product, a small amount of carbon deposition was observed, but carbon formation decreased with increasing the space velocity. On the other hand, with reduced monometallic Ni(10 wt%)/La2O3 catalyst, yield of synthesis gas and carbon decreased with increasing the space velocity.