A new grain boundary damage parameter, D-parameter, is proposed to evaluate grain boundaries damaged and to estimate the remaining life of components under creep loading at high temperature. This method is significantly different from the others firstly by specifying the grain boundaries normal to the principal stress direction in the area and secondly by defining the grain boundary damage by coalescence of voids. As the damage is analyzed from the surface structure of components, the replica is inspected by a SEM. The D-parameter method was practiced in in-situ observation of creep deformation for Ni based super alloy by SEM, and found useful for structural materials in which fine voids nucleate, grow and coalesce at the grain boundaries and cause an intergranular fracture.