Creep-rupture properties of a cast type Superalloy, Mar-M247LC. were studied in 760℃ and 980℃. The shape and size of γ′ particles in grains were known to have significant influence on the creep behavior and rupture life in relation to the micro-mechanism of plastic deformation. It was found that the creep properties were also affected by the cast structure which is composed of dendrite matrix and eutectic boundaries. The hardening effect of boundaries by solute segregation and eutectic phases may improve the creep resistance of grain or eutectic boundaries during creep-rupture.