In the ‘High Temperature Materials 21’ project, fourth generation single crystal (SC) superalloys have been developed adding Ru as a phase stabilizing element. A relationship between the amount of TCP-phase and the creep strength has been investigated for a series of Ni-base SC superalloys with 2 to 5 mass% Ru; these alloys were designed using an SC superalloy containing 5 mass% Re as a base alloy. The amount of TCP-phase decreased with increased Ru content from 2 to 5 mass%. and the amount of γ′ envelope, which normally forms on the TCP plate, decrease also with increase in Ru content. The creep strength increase with these structual changes but the main reason was considered to be the decrease in volume fraction of the γ′ envelope rather than the decrease in the amount of TCP-phase.