Ni-base single crystal (SC) superalloys with superior mechanical properties are being developed to improve thermal efficiency of jet engines and land-based gas turbines. The benefit of Ir addition to Ni-base SC superalloys and possibilities to design next generation Ni-base SC superalloys containing Ir were investigated in this study. Five equations that predict the tensile creep rupture life at test conditions of 800°C and 735 MPa, 900 °C and 392 MPa, 1000°C and 245 MPa, 1100°C and 137 MPa and 1150°C and 137 MPa were obtained for Ni-base SC alloys, including Ir containing compositions, with excellent multi-correlation coefficients that range from 0.95 to 0.98. From the regression coefficients of these prediction equations, we successfully clarified the predominating creep strengthening factors in middle - higher temperature and middle - lower stress creep test regions. Additions of Ir were not found to improve the creep strength of alloys, but were effective in suppressing the formation of topologically close packed (TCP) phases. The development of next-generation SC superalloys containing Ir with excellent creep strength was possible by using an updated alloy design program at the National Institute for Materials Science (NIMS). The developed alloy shows an improvement in temperature capability of 20°C in terms of creep rupture life over that of sixth-generation Ni-base SC superalloy TMS-238.