High strength single crystal (SC) superalloys for jet-engines and gas turbines have been developed in the “High Temperature Materials 21” project. In the context of this project, the authors previously developed the world strongest superalloy TMS-162 using an alloy TMS-138 as a base and adding Mo as a strengthening element as well as Ru as a phase stabilizing element. In the present study, Ta, Nb and Re were examined as possible strengthening elements instead of Mo. Ru was also added to stabilize the microstructure. Creep tests of the SC samples showed that Re had a similar effect as Mo for strengthening the SC superalloy, whereas the effects of Ta and Nb additions were not so significant as those of Re and Mo. These were attributed to the effects of the elements on the lattice misfit; Re and Mo are known to be partitioned preferentially to the γ phase rather than to the γ′ phase so that the aγ-aγ′ misfit increases. This is considered to be the reason for the excellent creep strength of the sample with the Re addition.