Nickel-based　single　crystal　superalloys　exhibit　excellent　high　temperature　mechanical　properties　due　to　their　unique　gamma　＇-gamma　dual　phase　microstructure.　Microstructure　design　is　the　major　strategy　for　increasing　their　properties.　However,　the　design　strategy　for　the　size　of　the　gamma　＇　phase　is　still　lacking.　This　work　investigates　the　effect　of　gamma　＇　cuboid　size　on　the　creep-rupture　life　of　nickel-based　single　crystal　superalloys　via　a　comparative　experiment　of　two　alloys　of　similar　gamma　＇　phase　volume　fractions　and　different　elemental　compositions.　The　alloy　with　smaller　gamma　＇　phase　cuboids　exhibits　a　creep-rupture　life　twice　as　long　as　the　alloy　with　an　average　cuboid　size　30　%　larger.　In　addition,　a　smaller　average　gamma　＇　phase　cuboid　size　also　contributes　to　a　lower　density　of　the　dislocations,　a　smaller　spacing　of　the　raft　and　lower　content　of　the　topologically　closed　packed　phases.　The　reasons　behind　these　different　mechanical　properties　and　microstructures　are　discussed.